What size screen do I need for home theater to project both 4:3 and 16:9 ratio pictures. Large as po

The answer to the size question turned out to be smaller than I had originally thought. The link below takes you to a "Viewing Distance calculator" which is just as useful for determining screen size if you know the viewing distance.

Before using is let's also consider aspect ratio and why the projector also matters. Two earlier respondents assume projecting 4:3 images within the bounds of a 16:9 screen. I actually do the opposite. I project widescreen (1.78:1 >> 2.4:1) full width on a 4:3 screen chosen to have a width optimum for my seating distance of 10.5 ft. I then have black bars top and bottom for all but 4:3 material ... which comes out really huge. BUT, this is only really practical if the projector has a native 4:3 format and reasonably high resolution. In my case my projector is a SVGA+ (1400 x 1050) 4:3 projector which delivers a 16:9 image via a 1400 x 788 pixel subset (about 10% higher resolution than a "normal" 1280 x 720 HD projector). However, a native 1280x720 widescreen projector would generate a 960 x 720 4:3 image -- much lower resolution than the Canon's 1400 x 1050 image, suggesting that with a widescreen projector the right choice for screen may well be a 16:9 aspect ratio (and projecting 4:3 images as a subset -- i.e. fill top to bottom, with bars on the sides).

Sorry if the foregoing is hard to follow, but I thought it worth including since it is important for a complete picture. That said, let's go to the heart of your question.