The size of my lights are what determines the size of my screens.
600w = 3x3 screen, no larger than 3.5x3.5. 600w in 3x3 screen gives 66.7 watts per sqft, in 3.5x3.5 you're just shy of 50 watts per sqft.
1000w = 4x4 screen, no larger than 4.5x4.5. 1000w in 4x4 screen gives 62.5w per sqft and in 4.5x4.5 it gives just shy of 50w per sqft.
I like 3 inch sized holes personally, but the size of your holes is a matter of preference I think.