Couple of thoughts on top of the advice already given (some overlap, obviously). All represent my own views and prejudices (I’m primarily a straight shaft paddler).
Straight v. Bent: I’d say bent, particularly when used with sit and switch technique, is more focused on the destination while straight a bit more on the journey to get there. Bent is more efficient on the straight (your experience may vary), straight provides more flexibility is stoke selection, blending strokes and adjusting to changing circumstances.
Paddle length: The techniques described all work for getting close - actual experience will let you fine tune (and your first paddle will make a great spare). As a bit of background: the goal is to have a shaft length that puts your top (grip) hand at about shoulder height, your lower (shaft) hand at about the gunwale, and the extends enough below your shaft hand so the blade is fully in the water. Blade length and shape are focused on what you’re doing with paddle in what kind of water. So size for shaft length and select the blade for what you’re doing. Your challenge is manufacturers tend to provide overall length, which really doesn’t do you much good for fitting the paddle. Give’m a call to find out shaft lengths.
Paddle selection: Well, you always carry a spare, right (up the creek without, etc.)? So mix it up a bit. Even large rivers and lake have shallow areas, and smaller rivers can get really thin. I generally launch with a beaver/otter tail (longer blade) and a Sugar Island (shorter wider blade). And if I know I’m going to end up in really shallow stuff or weedy backwaters I also bring a pole.
Oil v. Varnish: One thing not mentioned is the paddle finish. Some paddles are varnished, some have an oil finish. I prefer oil myself. For me it’s easier on the hands - I’ve never blistered with an oil finish, but have multiple times with varnish. If you fall in love with a varnish paddle, but have issues with blisters, sandpaper is a wonderful thing - I’ve sanded the shafts of two varnished paddles and converted them to oil with no problems.
Oil: Badger Paddles uses food safe hemp oil on their paddles. I tried it on my paddles and it works well. (google milk paint hemp oil). One side benefit is that it doesn’t contain solvents, so you don’t need gloves or to worry about spontaneous combustion of the rags.
Big blades: What I find interesting (applies to kayaking too) is that cultures (open ocean Bering Strait and Greenland Inuit) and working paddlers (voyageurs) who were focused on paddling to live or for a living, and covering distance primarily used narrow blades to get the job done. And it wasn’t from lack of wood (Inuit had access to significant amounts of drift logs and had other craft - umiaks - that were propelled by large bladed oars). It’s more about wear and tear on the body over time.
Well, hope that’s helpful and for sure your first paddles won’t be your last paddles, so experiment and don’t lock in too early. Remember, there are only three goals for paddling: go out, come back, and smile.