There are many factors that result in selection of the proper screen size. The two more important are the physical dimensions of your room (in meters, H,W,D), and the light output of your projector. For example, if you only have 2.5m ceiling height, you screen height will be limited to about 1.4m (and this will dictate width by aspect ratio). By the same token if your maximum seating distance is 3m, then there's going to be a limit on screen width (about 2.4m). On the other hand, your projector may not have enough light output to produce 8 Ft.Lamberts on a 2.4m x 1.4m screen.
The amount of ambient light has zero impact on the choice of screen size ... it certainly has an impact on whether or not you should do the project in the first place. Ambient light will alter the dynamic range of the reflected image. Recall that black is the absence of light, thus any ambient light will significant affect black level regardless of how bright your projector may be.
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