That's one of those subjects that everyone has a different answer for, but as far as I'm concerned, if you need the extra space for physical reasons (size, handicap, etc), you're perfectly entitled to it. The only time I get snarly is when the architect put the baby-changing table in the accessible stall, and even then I try to snarl at the designer, not at the parent. (This does not always happen, because sometimes parents take 20 minutes in the damn accessible stall while I sit waiting and needing to pee, and it is hard to remember to be gracious in a case like that.)

But seriously, needing to use the accessible stalls for issues of size is one of those things that if you ask five people about, you get seven opinions. As far as I'm concerned, it's a perfectly legit usage, though.