I think you usually can request a seat with more leg room, and it doesn't hurt to ask ahead of time about accessibility options, like a foot rest.

The sooner you book the flight the better chance of getting a preferred choice. If you do it online you can usually see the plane's seating layout and which seats might be taken already.

My dd flew to NY and her bf got her a front row seat with plenty of leg room, just so she wouldn't be crowded.