The overall CR (contrast ratio) will be crushed as the screen gets bigger, and the eye will have less light to deal with per sq. foot of screen. The screen will require less grey in it, as the light output won't support the grey required to turn that 'grey level' to a 'black level'. Remember, the black level is mostly perceptive in nature,and has very much to do with:
2)contamination of the room from projected light from the screen,
3)reflection of light back on to the screen (from the room, or originally from the screen..getting back to it),
4)cross reflection on the screen surface itself,
5)reflective gain characteristics of the screen itself,
6)contrast ratio of the PJ itself.... and how linear that aspect is
7)max lumen count of the PJ in question
8) how many feet of screen you are using
9)YOUR eye's (not someone else's eyes!!) dilational (responsive) behavior to the given light situation.
10)and, of course, proper set-up of the above mentioned screen
11)Linearity of the internal components (in terms of Gamma, brightness, and contrast scaling...all rolled into the entire equation of your projector-eye-screen/type/design/greylevel -room-setup.
No simple answers there, but the criteria for getting the right screen should at least be known.
The perfect screen for a given situation, for a DIGITAL (ie, not perfect black level PJ type) projector is MUCH harder to get to than that of a CRT type. Each individual situation is different, and no one single screen design will do. Granted, some grey will help, the question is that.. how do you get to the perfect level of grey for your particular situation(so as to maximize the characteristics of your given PJ?
Answer: through experimentation, and understanding.