For me, the bigger the better. I like the feeling of actually having to move my whole body while aiming the guns.
I recently bought a really cheap 80cm (31") "fishbowl" CRT TV and played a few PS2 gungames on it (classics like Virtua Cop as well as the new Time Crisis sequel) and it was great. Beats the hell out of playing on my 51cm (21") TV, even if the scanlines are more noticable.