You can pick the distance on your own for your projector, as most projectors work in a certain recommended distance range. Moreover, many projectors, such as a short-throw projector (which requires only a couple of inches from the wall) and a long-throw projector(which requires a long distance from the wall), have a certain range.  Therefore, first, check the type of your projector and then its recommended distance and screen size.  Most projectors accept a distance range from 9 feet to 11 feet for optimum image quality. Considering it a standard range, most popular projector companies design their projectors to work in this range.