I use a 6' wide screen in 4:3 aspect ratio, with an XGA resolution projector. I use three identical MTM speakers tilted slightly upwards and just underneath the screen. In a 14X13 room I have one row of comfortable seating with three seats in the audio "sweet spot", about 10 feet back. This is a front room with fireplace and other furnishings, not a dedicated home theater space, where a larger room would give you more than one row of seating.
Read the links above, especially the second one, until you have a good grasp of "subtended angle".
Now apply the MPAA guidelines to the rear row of seating in your theater: the horizontal subtended angle should be 30 degrees or more, and the vertical subtended angle 15 degrees or more. Depending upon the aspect ratio of your screen, these distances could be different, but the intent is to meet or exceed BOTH subtended angle minimums in the rear row. The MPAA guidelines are not sacred, but they ARE based on the polled preferences of movie-goers, in a large study with tens of thousands of subjects.
To establish the minimum tolerable screen distance for the front row of seating, you need to find the minimum distance at which the picture structure becomes observable. In the case of a CRT, this is where you begin to perceive scan lines. In the case of a digital projector, it is where you perceive the pixel structure. You need to be back behind this limit in the front row. If as would seem to be implied by your message you have not yet picked your projector, these minimums are approximately 1.2 screen widths for a 16:9 screen, and 1.5 screen widths for a 4:3 screen (assuming a CRT projector with a tripler/quadrupler or an XGA or better digital projector, and a viewer with vision corrected to 20/20).
Once you have established the above minimum and maximum viewing distances, you can figure out how many rows of seating will fit into your space. At this point, you select the projector location and throw distance to acheive the screen size selected. If you have not yet selected your projector, let me interject that for me, the most "theater-like" experience is a long throw projector on the rear wall, not a monstrous CRT hanging from the ceiling in front of me - just my preference.
There are not such cut and dry formulas for audio - the dispersion of sound varies greatly between speakers. But tying the sound to the screen is of utmost importance. It is critical to keep all three front speakers in the same plane, and they should be identical speakers, not two large towers and a smaller horizontal center channel - the horizontal speakers are sonic compromises for RPTV and direct-view applications. For front projection, use three identical speakers in a row with the RF and LF near the screen corners, and the CF right in the middle. They can be above or below the screen (above is preferable for more than one row of seating) and the tweeters should be aimed exactly at the listener's ears. Avoid the room corners if possible, as the two nearby walls and the ceiling or floor will over-emphasize bass, good for a subwoofer but not good for one speaker in an identical set of 5 or 6 or 7 speakers.
[This message has been edited by Gary McCoy (edited 09-28-2001).]