The general starting point is displacement.
How much do you weigh?

Usually a bigger boat will be necessary for a bigger paddler, but gigger does not always mean longer and/or wider.

Some of the more zoftig paddlers prefer short or narrow boats. When they get into such vessels, however, the boat sinks lower in the water than the original designed waterline. This lowers the center of balance (or center of gravity if you prefer that term) and makes the boat feel more stable.

Boat designers usually figure this will happen to some extent with varying loads, so they may calculate how much extra weight it takes to sink the boat an inch deeper than the design waterline, or they may calculate how many inches deepr the boat sinks when a fixed amount of weight (say 100 pounds) is added to the designed displacement. You'll find metric equivalents for all these measurements in designs done by and for people in countries that use the metric system, of course.

The distance the top of the sidewall of a boat rises above the water is called "freeboard". A good plan should have some mention of the freeboard when the boat is loaded to its designed displacement and floating at its designed waterline. As you add weight beyond the design displacement the real waterline rises, and the freeboard disappears.

Everything is fine until the boat sinks deep enough for water to come in. Then you have real trouble. The fix for this disaster is to build boats with higher sidewalls.

For canoes, which have tiny decks and are mostly open, I've seen a lot of designs with have 6 inches of freeboard. I don;t know if this is some standard or not. That amount seems to be okay on a mild pond, or a lazy river, but not for "big" water. For example, when my 14 1/2 foot canoe is used for day trips two large adults fit fine. But, when we added enough food and gear for a week of camping--about 120 extra pounds of it--the freeboard dropped to about 3 inches, and we had some serious worries.

On a kayak freeboard isn't the same issue. Some kayaks are designed to have parts of their decks submerged at times. A high coaming around the cockpit opening--and perhaps a really high, tightly-fitted sprayskirt keep the water out of the boat. If you can picture this, a kayak with a coaming 6 to 12 inches high might have the entire deck submerged, but with that chimney sticking out of the water, like the conning tower on a submarine, it would not fill with water.

With either kayak or canoe you want a boat which will not be so high out of the water that it is difficult to reach over the boat to get the paddle in the water. The height of your body, and the width of your shoulders are personal measurements which affect how any particular canoe or kayak design will fit you. The easiest way to chose a design is to try out as many boats as you can. Unfortunately that can be difficult to do, and you get reduced to making a "best guess" about which design to chose.

Hope this helps you make your choice.

If you want to post some details on how much you weigh, how tall you are, etc., then someone else who is your size may be able to post their experiences with either of the designs you mention. An opinion from someone much smaller or larger than you would not help you very much.

PGJ