Check out the screen/projector calculators over at ProjectorCentral. You can look at a lot of projector models, find the screen size you want, and it will tell you the throw distance. Keep in mind that the closer to the screen that the projector is, the brighter the image will be, and the more ambient light that you can have. May not be a factor, but just keep it in mind. Some projectors have to be closer than others and some have to be further away. You should have a lot of choices to choose from.
Here is a link:
ProjectorCentral
