Join the Met: Live in HD
The Met's criterion is HDTV. That means no less than 1280x720 native in a 16:9 aspect ratio (prefer 1920x1080). The scaling engine should be able to scale 1920x1080 interlaced to whatever the projector uses.
The more contrast capability, the better.
The projector should be able to deal with a gamma of approximately 2.5 (anything between 2.2 and 2.8 should be okay).
Prefer white-area screen brightness of no less than 16 ft-lamberts. To calculate ft.-lamberts, divide the projector output in lumens by the screen area in square feet and multiply by the screen gain (a flat white wall has a gain of 1).
Please note that the Met signal is 16x9 1080-line interlaced.