Can the projector be 20 ft away from the screen?

Attainable???...Yes. Practical???...Here are the issues.
You didn't specify which projector you would use. Knowing the projector would indicate the lens ratio. A simple equation is to divide the throw distance by the lens ratio and you will get your screen width. Assuming a 20' throw (LENS to screen) and a lens ratio of 1.5 (just a general assumption), your screen width would be 13.3'. This gives you a screen height of 10', assuming a 3 x 4 aspect ratio. Since we don't know how bright the projector is and how much ambient light is in the room at any given time, there are more factors to explore. I realize I made quite a few assumptions, but if you fill in the missing info I can make some suggestions.

Rayd

Rayd, I'm hoping to get suggestions here for a used projector under $5000. There is only one window in the place, it is 12 ft by 12 ft at one end of the room. I have a window covering that will make the room pitch black. Other than the window, there is no other external light source. I've just began my HT quest and I have no experience with projectors at all. All I know is that I have the room for a big picture and I don't want a big screen tv console mucking up my 2 channel setup. I have to do some research to understand things like lens ratio, ambient light and other projector nomenclature.

Mitch, like any audio component you can audition at home, have you considered auditioning a projector in your room? You didn't mention if you were interested in LCD, DLP, CRT, ILA, etc., or if you were buying used from a dealer or from Audiogon, but if you can audition a projector in your room you will gain answers to many of your questions. Many LCD and DLP projectors are small enough to make an audition quite simple to do. I applaud you for wanting to do your homework.

Rayd

I asked about what the equip. was because the type makes a huge difference. If you wanted CRT as opposed to lcd or dila all of this would factor into both screen size and distance. Each of these projectors depending on tube size or spec. has a fairly precise mathematical formula for determining distance to screen size. Also picture quality should be of parmount importance and CRT by a small edge still has the best overall picture quality, but light becomes a greater consideration. Another question is budget. It seems that your application would call for a DILA or LCd because you can generally use these at greater distance from the screen with larger screen size and still maintain decent quality. Dila is the latest and greatest and expensive. 9" CRTs might meet your needs with a 16:( 110" or greater screen. The prob. there is that a lot people have big bad screen envy and sacrifice quality for size. Big screen translate generally into quality degradation. Bottom line whats your budget? How picky are you? ECT.

Yes, DILA for sale on avsforum for 3500 or so from DVoss. I had the unit first and it is a great unit for 120 inch or so screen. I believe it will work from 20 feet or more.