I am not 100% certain about the legality behind this but it has worked for me dozens of times when I travel, particularly on long-haul flights across the pacific. There are certain types of restrictions like you can be a giant fat-ass and you also can't be a 110 pound woman or featherweight UFC fighter.
You also can't have a child with you or a pet (which most airlines don't permit anyway) and there are other restrictions that I will get into shortly.
Basically, as far as a flight attendant friend of mine is concerned and I am sure she is not involved in the upper management of the airline that she works for; the airlines are not actually allowed to sell these seats because the requirements are that the person who sits in them needs to be capable of helping should the plane crash in which case well, everyone is going to die anyway so the size requirements of the person in the seat is likely not going to be much of a factor anyway.
I'm not saying that it has never happened but let's go ahead and be honest here. You are in a metal tube that is traveling 500 miles an hour, there is very little chance that you are actually going to have to take on the responsibilities of the exit row team and if that situation actually did arrive, well, i hope you would do so but I think most people would likely just fend for themselves.
I am not writing this to make anyone anxious but I am saying that if you are traveling in coach / steerage / poor people class the best way to ensure that you are going to get just a little bit of extra space (it is actually quite a significant amount of extra space, the only thing you need to do on check-in is ask for an exit row.
Since (according to my friend, who is in no way an authority on this subject) there are always going to be availability in these seats. She has told me that these seats serve as a sort of "overflow" for when they intentionally oversell flights to maximize profits and will re-arrange the seating arrangements so that they can fill, I dunno, an extra 20 seats or so on the planes.
The only way you are ever going to get these seats is if you ask for them though and in the dozens of flights that I have been on I have never been denied one of these seats. The only downside is that you are not allowed to take your shoes off, or at least you can't do it when the flight attendants can see you. I always take my shoes off and cover myself with a blanket just like everyone else does.
It aint exactly business class, but it is a HUGE step up from the regular seats that are back in poor-people class and all you have to do is ask for it when you are checking in. Just make sure that you are a normal sized human and have no kids with you and the answer will always be "yes."
That has been the case for me for nearly 20 years and wow do I enjoy the extra leg room.