Really depends on the resolution your PJ is capable in the quality of the scaler. a 100" 4:3 screen is 60x80. 12' is 2.2x the height. Still falls in range. A 16:9 100" screen is about 45" in height. At 12', that is over 3x picture height. While this will certainly minimize scan line visibility, it also places you pretty far back in terms of included viewing angle.
Most here would tell you to go 16:9 and use a constant height picture instead of 4:3 and having a constant width. Really depends on the % of 4:3 (TV) vs. DVD you intend to watch.
Personally, I am still considering 4:3. At least that way, when I "burn" the tubes by watching 4:3 (mine will be about 50/50 unless someone builds a good scaler with non-linear stretch for an affordable price), the burned area will eventually be out of the picture, so to speak, with the increased amount of 16:9 television. A 100" 4:3 screen, when masked to 16:9, still yields a 92" diagonal screen.
I'll probably catch some flack for this line of thinking but it does deliver a big picture in both ratios.