Setting up screen size on BD Player

[RockyIII]

I own two DLP projectors (Optoma and Vivitek). When I connect my bluray player and I go to Video Settings, it asks me to enter Screen Size. How do you deal with that, since the player is not obviously connected to a TV of a fixed size? Do you leave Screen Size settings at 0 or do you try to find as close a size option as you have your display set up on the wall??