That sounds about right- and also depends on your viewing preference (esp wrt watching Cable/SD). With HD you can go as near as 5-6 ft from your 60-64" HD set. For piss poor standard definition you have to step back to 8-10 ft depending on your viewing preference! I think 60"+ will be one for you to begin..(then it will shrink and you will want a bigger set..hehe)
If it was just HD or DVD viewing yeah 100" would do well - go FP then

Cheers,
Kishore