I should have specified that this will be an HDTV situation, but already Im seeing great food for thought here, as expected.
Obviously, I'll do my best during my demoing to take into account distance as best I can. It gets tricky, as oftentimes the store situation is vastly different from what the home situation would be.
Crutchfield's A/V Advisor lists a chart like this, and based on my experiences, it seems fairly close to what I would expect.
Standard (analog) TVs with 4:3 aspect ratio
Screen size Suggested viewing distance
27" 6.75 ft
32" 8 feet
36" 9 feet
40" 10 feet
45" 11.25 feet
50" 12.5 feet
55" 13.75 feet
60" 15 feet
HDTV-capable TVs with 4:3 or 16:9 aspect ratio
Screen size Suggested viewing distance
30" 6.25 feet
35" 7.3 feet
40" 8.3 feet
45" 9.4 feet
50" 10.4 feet
55" 11.5 feet
60" 12.5 feet
65" 13.5 feet
By this, going 60 would put me "over the top" as it were.
