The paddle size and shape depends on how far it is to the water, and what you want to do once you are there.
Start with holding the shaft over horizontally over your head, as if you were a rifleman wading across a river. Your upper arms should be horizontal, and your fore-arms should be vertical, so that your elbows are at 90 degrees. Without changing the position of your hands, start paddling.
If the blade is fully buried in the water but the shaft is not in the water, then you have the right length. If the blade is not buried, then get a longer shaft. If the shaft is entering the water, get a shorter shaft.
If you want to paddle at a faster pace, or if you are concerned about overworking muscles, get a narrower blade. If you want to paddle at a slower pace, or if you want occasional power on demand for wild water, get a larger blade.
The longer your reach during the set-up, the longer the blade. The shorter the reach, the shorter the blade.
Keep trying various shaft lengths and blade sizes/shapes until you find something that works for you.
In a flat water kayak, you either sit high up but close to the gunwales (e.g. a sprint racing kayak – you squeeze in between the gunwales and sit about four or more inches up from the hull) and use a powerful high angle stroke (with the paddle shaft more toward the vertical than toward the horizontal) with an average size paddle (about 205 cm or so), or you sit low but somewhat further from the gunwale (e.g. a sea kayak – you are a couple of inches or so from the gunwales and sit two or three inches up from the hull) and use a less powerful low angle stroke (with the paddle shaft more toward horizontal than toward the vertical) with a long paddle (225 cm or so). The lower the angle (the more the shaft is toward the horizontal), the longer the shaft must be before the blade reaches the water. (Please don’t take the lengths as gospel. They are only approximations, and will vary depending on quite a few factors). (And never use this many parentheses in a paragraph.)
A big benefit of a high angle stroke is that it is a pure forward stroke with no sweeping action. Another big benefit of a high angle stroke is that it lets you use your powerful shoulder muscles to hammer down with the catch and follow through like a scythe. The high angle stroke is quite similar to a canoe stroke, only with a lot more trunk rotation. The problem is that you have to be fit to keep trucking along with a high angle stroke for a long time. Thus most recreational kayakers use the less efficient but less exhausting low angle stroke, which is more akin to wiping a table with both hand at the same time – a lot less powerful, but a great deal easier.
The only way to paddle a canoe or kayak while both sitting high and fairly far from the gunwales is to use a very long paddle (about 245 cm or so) with a less powerful low angle stroke. You can’t use a high angle stroke because you are too far from the gunwales, so you are stuck with a low angle stroke, but because you are sitting so high, the low angle stroke forces you to use a very long paddle to reach the water.
Using a very long paddle with a less powerful low angle stroke is very inefficient. First, by the blade activity taking place so far from the boat, the stroke is more of a sweep than a forward stroke. Second, although trunk rotation remains, you can not drive down and add a lot of oomph to the rotation with the powerful shoulder muscles.
Its the getting back in that is difficult.