With progressive scan, a seating distance of 1.5-2X screen width works well for DVDs.
4:3 diagonals are 5/4 width; 16:9 diagonals are 18/16 width so diagonal measurements which meet this would be .625-.825X seating distance for 4:3 sets, and .563-.743X seating distance for 16:9.
For contemporary domestic arrangements, this probably means "as big as you can afford."
Interlaced NTSC gets unwatchable on much smaller screens.