Promoting some comments to an answer
There are various things you might want to consider for your seat:
- Some seats are near the galley (can be noisy) or toilets
- Some seats have the tray table or screen in the arm rest that can reduce seat width
- Some seats aren't aligned with a window
- Some seats have extra leg room in front of them
- Some seats don't allow you to put baggage on the floor / under the seat in front
- Some seats have extra storage space alongside
- Within a given airline, some planes can have slightly different sized seats to others within the fleet (eg the 777 planes might have different seating to the 319-321s)
The good news is that there are websites that track all this, who can let you compare seats between airlines, and identify good or bad seats + why!
These sites include, but are not limited to, things like Seatguru.com and Seatplans.com. Search around for sites like that, see which one(s) you get on with, then check those.
Also, different things matter to different people, so be sure to read the full details. Someone might say "by the toilet, bad, mark as red", while someone else with bladder problems might decide that that seat is therefore the best on the plane and shout be green!