Seating in airplanes is "flexible" since the seats attach to the floor rails on extruded metal bits that have holes at closely spaced intervals. An airline can change the "pitch" between seats pretty much at will, and even knowing the specific model or airplane you intend to fly on doesn't give a lot of assurance of how much space you'll have.

As a general rule, if you get the "front seat in the class" there won't be a seat in front of you, so many times there'll be a little more room to stretch out. Also sometimes, the rearmost seat in the class also will have more leg room, but you may not be able to incline the seat, and the bulkhead behind is probably a lavatory.

Nearly always the seat rows with the emergency exit doors will at least assure you that you'll have as much leg room as anyone else in the cabin class. In an airplane set up to ordinary spacing, the exit seats may be a little longer in pitch. In "higher density" layouts they'll be the same as all the others. In "very high density" layouts it may be necessary to leave them a little longer in the leg than the others, since there has to be at least the theoretical possibility that the fat people won't jam up in the exit.

IF you can get a seating plan for the airplane you'll be on, it may be some help; but the airlines - especially recently - don't seem to much care whether they give you any reliably helpful information.

Your most likely source for decent information is the particular airline(s) you plan to fly on, and they may be able to send you something or may have something online, but my experience with what they publish hasn't elicited a lot of trust.

John