That’s not true, Theodore fell off at age 29. That said I think it probably had more to do with the fact the game totally changed after the ‘05 lockout and he didn’t adjust and had some bad injury problems rather than his size.
Regarding Theodore, he definitely fell of compared to his absolute peak. Advanced metrics suggest he performed fairly well (not "elite by any mean") in his later seasons though, having some seasons where he performed exactly how you'd expect a typical starter to perform. He still had that period he was pretty bad after the lockout. I'm sure, as you say, the "fell off" after the lockout had nothing to do with his size, too. Toskala seems like a case similar to Theo although his pre-lockout sample size was small and he never really "rebounded" like Theo did.
My entire point was mostly to show the poster I quoted that there's nothing to suggest that established smaller goalies are more likely to fall off faster and more drastically. If anything, I wouldn't be surprised if a thorough analysis showed that large goalies (says 6'3-6'4+) are more likely to fall off faster. Smaller goalies, in order to establish themselves, might have to better master the technicalities of the position as opposed to larger goalies who might rely more on their size to block shots.