As big as you can go without compromising picture quality. I would personally go for 64/65 at 12 feet. I sit 6.5" away from my new 55" and while I love it, I wish Panasonic made it in a 60". I think the 65" would have been too large for me.
ht Panasonic TC-P60ZT60, Monitor Audio: Silver RX6, RX Centre, Radius 90HD; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Monitor Audio Silver RX1, REL T3, Apple TV, Peachtree Audio DAC•iT
lr Panasonic 50ST60, Sony BDP-S5100