It depends on the application. If you travel a lot, your projector should be small and lightweight. If the projector will be stationary or ceiling-mounted, size doesn't matter as much. Large projectors typically offer more features and better image. Another consideration for fixed projectors is mounting location and necessary image size.
Image size depends on the distance from screen to projector and projector lens.
In many cases, the projector is conveniently located at the rear of the room or is ceiling-mounted. In these instances, a stipulated image size is obtained by selecting a projector that provides the correct image size at the necessary throw distance. One alternative is to use a projector with the lens replaced to provide a longer throw distance. If you are at the back of an auditorium, this throw could be 50 feet or more. The amount of light, or lumens, remains the same, assuming image size has not changed.