Log onto any screen-distance calculator on the web before buying your projector. These calculators allow you to find the optimal distance from the projector to place the screen. They also give you the optimal screen size for the projector. Weigh these factors, including the room size itself, to decide upon a projector that will suit your space.
Determine the brand and make of your projector. Enter this information into the distance calculator. The calculator will display a 'throw range' for your data/video projector, which is the narrow range of optimal distances your projector should be from the projection screen.
Use the measuring tape to find the diagonal distance on the screen; the diagonal distance is the distance from one corner to the opposite corner. This means starting at a bottom corner, say on the left, and measuring across up to the top corner on the right. Write this distance down.
The information displayed will be in standard sizes. The buttons on the dimensions of your screen can be moved to right or left so you can taek measuremnts and hone down the distance calculator results.
Take the diagonal distance you wrote down and return to the screen-distance calculator on your computer. Below the 'Throw Range' will be a section called 'Screen Size.' There will be slidable buttons in this section. There will be one area for diagonal distance. (There will also be areas for horizontal and vertical distance, but these are just proportionate to the diagonal distance and are redundant). Slide the button on the area until you are at your recorded screen size. This value will alter the throw range above. Note this new throw range.
Find the distance between the spot in the room where you plan on placing the projector and the place (perpendicular and opposite the projector) where you want to place the screen. Compare this measurement with your 'throw range' value. If the value is smaller or larger than the 'throw range,' then you may have to move the projector, purchase another screen, or get another projector (keep your receipts!).