Thanks WhyNot. (44HH here…I shudder.) Your second link looks great–I’m close to but not at the upper limit of their sizes. Next week I’ll check out their order and return policies so I can do a try on. 
Have fun, and don’t worry about your size.
The better one’s overall physical fitness, the better one’s performance in kayaking, but at the entry level, as long as you have fun, it does not matter whether you are beating a four minute mile, or hucking waterfalls, or mysterying in the depths.
Lack of aerobic capacity and lack of muscle strength will limit how far and how fast you can paddle. Listen what Billdo has to say about using trunk rotation to power your boat, rather than using your arms. When the blade is in the water, rotate at the hips, waist and trunk to pull your body up to the blade while keeping your arms relatively straight. Imagine that you are scything wheat. If you are pulling with your arms (if you are bending at the elbow when the blade is in the water, then you are pulling with your arm), you will wear out your arms. The simple solution is to paddle at your own speed. Don’t try to keep up – if your paddling companion is a safe paddler, he will escort you at your speed, rather than play macho.
A large tummy will limit your ability to rotate at the waist, which in turn will reduce the effectiveness of your forward stroke. It will also be more difficult to roll up, for the easiest rolls (extended and screw) start with you tucking forward onto the front deck. Neither of these are considerations for novice paddlers, and in fact most paddlers never develop a good forward stroke and never learn to roll, but still enjoy many years of happy paddling.
Boats are made for a wide variety of body sizes, so finding an appropriate one for you should not be a problem. If your companion does not have one, just rent one from an outfitter. The same can be said for personal flotation devices (PFDs).
Always wear your PFD while in the boat, and keep it done up. Wearing one that is not done up will just make things worse if you swim. Same goes for a helmet if you are paddling in moving water or surf (and don’t go in either your first few times in a kayak).
Wear sun screen and bring sunglasses and a sun hat – water reflects light.
Bring a variety of clothing, so that you can keep warm even when wet.
As far as getting stuck in the boat goes, don’t worry about it. If you go over, (1) relax, (2) pull the rip cord if you are wearing a skirt (3) wiggle your bum out, (4) either slide backwards until your legs are out or somersault forward until your legs are out, and (5) come up for air. Folks only run into difficulty when they come up for air too soon, for that makes it a little more awkward to get out of the boat.
To avoid dumping, keep your weight centered in the boat – try not to grab the sides of the boat. Use the flat of your paddle to help balance you – spread the flat of it back and forth across the surface, the way you would spread peanut butter on toast, or in a pinch, smack the flat of it down on the water the way a beaver slaps its tail. Think of the boat and your paddle making a three legged stool, with the front of the boat one leg, the back of the boat the second leg, and the flat blade of your paddle pressing on the water the third leg – as long as you have three legs you won’t tip over. Try to be relaxed, and separate your upper body from your lower body. If you are tense, when the boat rocks your upper body will rock, and possibly lever over the boat. If you keep loose, and separate your upper from lower body, when the boat rocks, your upper body will still stay centered, and you will not lever yourself over into the drink. If you are feeling tense and worried about tipping, just think of yourself as a mermaid already swimming, with the boat being an extension of your body, rather than legs. Mermaids like swimming – there is nothing to be nervous about.
Stay on shore unless the water is calm, until you are more experienced. Practiced portaging make for perfect paddling. I assume that your companion is the sort that will keep you from getting into any trouble. Just in case he is not, I strongly recommend that until you start feeling comfortable in a kayak, you should stay very close to shore (particularly if the water is cold – hypothermia kills swimmers), and not paddle in moving water or in surf. If you do end up in a current, keep well clear of any rocks, logs, or other obstructions until you can take off the river. If you are going sideways, lean downstream to avoid flipping. If you are about to pin sideways on something, throw your self onto it so that your head will be above water until someone un-pins you. If you are being swept out by the surf by a rip-tide or a river current, swim parallel to the shore until you are out of the current, and only then start swimming back to the shore. At all times have a plan for what to do if things go wrong.
Finally, as pointed out in the Hitchhiker’s Guide to the Galaxy, be sure to bring a towel and a sense of humor. Bring a really big towel, for paddlers strip down to change into dry clothes at the end of a day’s paddle. Bring a sense of humor, for a stripped down paddler at the end of the day resembles a white shivering prune.
Get wet. Have fun. When come back bring tales of delight and adventure.
Muffin
(wild water and sprint racing instructor)
For wild water and surf kayaking, the low-cut PFDs that WhyNot has pointed out are your best bet. If the PFD sits much below your rib cage, the kayak’s cockpit may get in the way. If the side panels are more than a couple of inches high, you may chafe. If the shoulder straps are more than about an iinch and a half wide, you may chafe. In other words, try on the lowest profile ones you can find, preferably spend some time paddling in them, or at least include lots of movement while sitting down while your try them. Don’t purchase one until you are very happy with the fit.
Once you are certain that you will not chafe, the next thing to be certain of is that the PFD should be snug against your breasts. If it firmly holds them down against your chest, you will be more comfortable as the day goes on. (Some folks may be wondering, “But Muffin is a guy!” Very true, but my 22 person dragon boat crewmates and my 6 person outrigger crewmates are all women, and all but three of them are breast cancer survivors. They have a wide range of body and breast sizes and types, and have gone to great lengths to find the best possible fitting PFDs.)
Another thing to look for in wild water and surf PFDs are shoulder straps that are made of webbing, so that when you are being fished out, the shoulder strap will not break.
If you are not venturing into wild water or surf kayaking, you should consider wearing an inflatable PFD (but check your state’s regulations). They come as belts, or as shoulder harnesses. No more bulkiness, chafing, or overheating problems. The belt ones are about the size of a large wallet. The shoulder harness ones are about the size of a small wrapped up towel draped over your shoulders.
The benefit of the shoulder harness model is that you don’t have to do anything once it is inflated, whereas with the belt model, you have to pull it over your head once it is inflated. The benefit of the belt model is that it is by far the least intrusive PFD, which can be a godsend if you have sensitive or just plain hard to fit breasts, for prior to inflation it is not in contact with your upper torso.
Inflatables come as either automatic inflating (by hydrostatic pressure – they inflate when you are in the drink, but do not inflate if you are splashed), or by you pulling the rip cord. If the system fails, you can inflate them on your own like blowing up a balloon. Note: there are also automatic inflating PFDs that are triggered by salt pills rather than hydrostatic pressure. These are not appropriate for kayaking, for kayaking can be such a wet sport that they may go off prematurely.
These inflatables will float you face-up, so that even if you lose consciousness (which can happen if you are in cold water for any significant period of time), you will still be able to breathe. The vest-type PFDs will not do this. They will keep you floating with a better probability of floating face up rather than face down, but with nowhere near the probability as the inflatables. That, by the way, is why PFD are called PFDs and not life jackets, for life jackets keep you floating face up.
Where inflatables are not appropriate is in wild water and surf kayaking, for they do not offer any impact protection, they may (depending on the model) require you to do something like inflate them or pull them over your head, and they tremendously impinge your ability to swim. Ever wonder why you never see a person on a surf-board wearing a lifejacket or a PFD? Surfers need to be able to swim and dive, but lifejackets and to a lesser degree PFDs make swimming more difficult and diving just about impossible.
A belt type manual inflatable: http://www.mustangsurvival.com/products/product.php?id=197
A shoulder harness type automatic hydrostatic inflatable: http://www.mustangsurvival.com/products/product.php?id=506