Here are some guides to selecting the correct screen:
MAIN CRITERIA:
1. The dimensions of the audience area.
2. The projection format or formats to be used

1. Audience Area
The goal is to make the screen large enough so those in the back row can see the subject matter easily but not so large as to overwhelm the closest viewer.
Width - The generally accepted guideline for screen width is the Two (2) and Six (6) Rule:
A. Screen width should equal the distance from the screen to the first row of seats, divided by two (2).
B. Screen width should equal the distance from the screen to the last row of seats divided by six (6).
C. In case of difference, the guideline yielding the larger size should prevail.
Height - As a rule the screen height should equal or exceed the distance from the screen to the last row of seats - divided by eight (8). Ceiling height and projection format may affect this decision.
Ceiling Height - The bottom of the screen should be approx. 1.2m above the floor so that all viewers are able to see the bottom of the image.
Projection Formats - When a screen is to be used with two or more different formats use the above rules to determine the height and use the aspect ratio to find the greatest width that will be used by any of the projectors.

2. Common Projection Formats
Video/Computer Format 1:1.33 (4:3), HDTV 1:1.78 (16:9), Widescreen 1:1.85, Cinemascope 1:2.35, Audio Visual (or slide/overhead) 1:1. Basically the projection screen format should match the native aspect ratio of your projector. Multi format screens with motorised masking are available but fairly expensive.