What size screen do you want? What size is the room? You can usually download a projector manual and in the installation chapter it will explain the formula for what size projected image you can expect given a particular distance from projector to screen.
Usually both the projector and the source device (cable box, DVD player) have settings to make sure that the projected image matches the source (i.e. a 16:9 movie displays in 16:9, a 4:3 TV show displays in 4:3).
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