The right screen size will give the most comfortable viewing. This is calculated based on how far away you are sitting, and remember! Screens are measured in width, so you need to be sitting 1.5 to 2 times the width of the screen away.
All projectors have different throw ratios. This simply determines the minimum & maximum distance from the projector to the screen. For the correct distance for your projector please look at the specs for your model.