Firstly, consider what the Projector will be used for- is your priority video or data. If video is your main criteria, Sanyo have a range of specialist Home Theatre units, the PLV models. For data Projectors, consider how big the room will be and how many people are going to be in the audience. These factors will determine the screen size. You will then need to consider how much ambient light you will have to contend with. As a rough guide under normal office lighting, 750 ANSI lumens per square metre of screen should give a bright and well-contrasted picture.