I got a great deal on 2 matching wood chairs at an outlet store. One is 30" bar height, and the other is 26" counter height. I’m going to take 4" off the bar height chair so that they match.
I’m sure I need to find someone who has a power saw to do this. Otherwise it seems like it would be a pretty straightforward job.
Are there any tricks to getting the cut even so that the chair doesn’t wobble? Any advice or cautionary tales?
The biggest advice I can give is “measure twice, cut once”.
As above. And the second adage - always cut on the waste side of the line. (This means that there is no concept of cutting on the inside or outside, and certainly never on the line.)
Also, I would never consider a power saw for such a job. A hand held tenon saw would be my tool of choice. Vastly more control and much safer. It is a pretty exacting job, on a hard floor even a fraction of a millimetre is enough to cause a wobble.
Seconding the tenon saw. They make power saws for trim if you can find one. The best advise I can give you is to get the right blade regardless the saw.
Assuming the legs are square in section, mark the cutting line all the way around - on all four faces - this way, you’ll ensure that the cut remains straight and square as it progresses through the thickness.
If they’re round in section, use a mitre jig to ensure that you get a square cut across the timber.
Cut with a hand saw, and if possible, clamp the chair down so that you can concentrate on sawing, rather than steadying the workpiece.
And then by a block plane and use it to fix up the wobbliness you are going to have after you finish with the saw 
These chairs will be on carpet, so that may provide a little bit of forgiveness in the wobble dept. The chair I’m going to cut already has a wobble, which I think is funny—I hope to eliminate it!
I said “power saw” as a lay(wo)man who knows not a lot about saws. I just looked up tenon saw on google—a hand-held saw? Or does the name tenon mean it has a different kind of teeth than the one I have out in the shed? The idea of sawing by hand gives me anxiety. My dad always said that if you don’t cut quickly while supporting the cut piece, you could splinter off wood on the good side. I know what this means, I’ve seen the results—this makes me nervous!!
These are square legs.
I imagined turning over the chairs on a table and putting the shorter chair leg-to-leg on each side with the taller chair and making my measurements that way. Is this a good idea? The shorter chair has no wobble.
I wonder if I should ask a woodworker to do it. I’m certain I’m capable, but I basically can’t screw this up, or the great deal I got won’t be so great anymore.
Here’s one way to eliminate splintering. After the marks are made all the way around the work piece, use the saw to make shallow cuts on the waste side of the line all the way around. Perhaps 1/16" deep. That way no splintering will occur at the finished surface. With small sections such as chair legs it’s unlikely anyway.
To eliminate wobble, there are screw adjustable glides that fit into threaded tee nuts driven into drilled holes in the end of the legs. If the chairs are going to be on carpet you’re unlikely to ave issues if the cuts are reasonably close.
Strictly speaking a tenon saw is a saw for cutting tenons - which for one half of a mortise and tenon joint (of which your chair will almost certainly use in its construction.) But they are common saws that are generally suited to reasonably precise work, and have a fine tooth pitch that will leave a clean cut. They have a stiffener that runs along the back that makes them stay perfectly straight when in use, which helps the precision. Because of the fine teeth, if used carefully they are unlikely to rip out the reverse side of the cut.
If you put together all the advice above you have a pretty comprehensive set of instructions on how to do it. Since the legs a square it should be reasonably easy. It is unusual for a chair to simply develop a wobble without some underlying issue, so I would want to have a look to try and understand why. It may be as simple as a missing pad under one leg, but the leg may be crooked, which would need fixing before sawing to length.
One way to mark the legs is to place the chair on a level floor, level the chair to remove any residual wobble by putting some packing under the shorter leg, and then measuring up from the floor to the point where you want to cut. This will ensure that the cuts are themselves co-planar. A neat trick is to use some object of the right width as a marking template, and simply place it on the floor, and use the other edge as a straight edge to mark all around each leg. A piece of anything flat and stiff with two parallel edge four inches apart. You should be able to mark the legs perfectly all around like this - and because you are measuring from the floor up the legs while the chair is perfectly level, the marked lines should all be perfectly level and co-planar.
Then, as described, clamp the chair leg solidly and cut so that the sawn edge just touches the waste side of the marked line. Watch that the saw cut stays in the right position of all sides of the leg as you cut. Keeping the saw perfectly upright and taking long straight cuts helps. Clamping the leg both avoids you needing to worry about steadying it whilst you cut, and yields a much better cut that is made with much less effort.
Practise cutting on some scrap wood first. Practise nice long even strokes where the saw stroke is perfectly straight and the saw stays perfectly vertical. It isn’t hard at all, but you don’t want to be learning how to control the saw like this whilst doing a critical cut.
Measure the height of both chairs so that you know how much has to be taken off of the tall one to get them to match. Let’s assume that it’s 4".
Put the taller chair on top of something that is flat, like a countertop or a table. If the chair wobbles, shim up the short leg(s) with paper or cardboard until the chair is stable and is as close to vertical as you can get it.
Make a marking jig by taping a pen or pencil to the top of a can or glass or something so that the marking part of the pencil is 4" (or whatever) above the surface that the chair is standing on. Use this marking jig to place a line completely around all four sides of each leg of the chair by sliding the can around the base of each leg. If the legs are splayed at all, the lines on two sides will not be perpendicular to the length of the leg, they’ll be at a shallow angle.
Use a fine toothed saw to make a shallow cut on all four sides of each leg, following the lines that you just laid out.
Complete the cut on each leg; the shallow cuts you made previously should prevent tearout.
I would recommend a Japanese handsaw of the type shown here. They have very thin blades and they cut on the pull stroke rather than the push stroke. They leave a very smooth cut surface and they don’t tend to wander, and I find them much easier to control - more of a scalpel than a chainsaw, like many of the handsaws you might typically find sitting in a shed somewhere.
ETA: d’oh! Should have read Francis Vaughan’s post above! Still, I’d recommend a Japanese handsaw over a Western style backsaw.
a larger rip or crosscut saw (wood or plastic handle, flexible blade 2 1/2 feet long, skilled person can play music on) is the wrong type of saw to use for this.
a miter saw would be good, it is a short stiffened bladed saw. you will likely find it called a miter saw in many stores and not a tenon saw. large home improvement stores will often have it on sale (with a plastic miter box) for maybe $5, regularly less than $10, this is good enough for occasional use.
I would say yes. As a designer I have occasion to buy furniture pieces and have them altered. I usually find a local woodworker to do this for me rather than have the contractor try.
BTW, power saws can splinter wood easier than a hand held saw in my experience.
Measure and measure again.
Hand saw.
Piece of tape on each leg.
It doesn’t need to be more complicated than that. You’re over-thinking this.
I hate threads like this. Every time I see one, I want to say “I’ll do it right now for 10 bucks. I have the tools right here in my van!”, but everybody who starts threads like this lives too far away, so there goes my easy 10 bucks. 
I asked on the work bulletin board if I could borrow a tenon saw from anyone. I got an offer from a woodworker co-worker (ha) to do it for me. Pressure’s off!
I told him he could make the cuts and I’d do the finish-up sanding.
Now, he doesn’t want me to pay him, but I’m going to do something for this guy. Any suggestions as to what would be appropriate compensation? I’d offered to pay him in cash, food, or beer. He declined all.