We recommend to put the projector about 10-15 feet back from the screen to get a very clear image. Depending on your actual setting you could put it closer or further away. Experiment until you get a clear image, at the best resolution.
It also depends on the kind of projector are you using. The throw distance is different for each.
I recommend that you go to the website to see the throw distance for your projector: http://www.projectorcentral.com/projection-calculator-pro.cfm
It definitely depends on the throw of the projector, for example a Benq W1500 is rated as a 1.07 to 1.71 zoom throw, So I multiply 144″(screen size) x 1.07 and then again x 1.71 and I know that I need to set this projector between 154″(12′-10″) and 246″ (20′-6″) away from the screen.
For more information please check out our guide setting up a great backyard setup
Or take a look at our recommended projectors and screens