I have heard of people buying smaller and regretting not going bigger, but aside from
the dude who bought a set too big to move into his apt, I've never heard of someone buying bigger and regretting not going smaller. There are some formulas about screen height and viewing distance that say 9.5' is too close, but those rules were for SD sets. Line doubling (standard in HD sets) allows you to sit much closer.
Here is a table to gauge HD viewing distance. So 57" at 9.5' is fine. You could get away with 65" at that distance. The general rule of thumb with HD sets is to get as big as you can afford/accomodate. Don't worry about viewing distances anymore.