i am wondering if it is possible to make the projector so that i can have a smaller screen size like 4 to 6 foot. i am going to build a pj with a 15" monitor but do not have enough room right now for a 8' diag. screen.

Yes.

do i need to have any different panel spacing or is it all done with the focas of the machine?

Is is simlply done by moving the PJ closer to the screen. The closer the PJ is to the screen, the smaller the image. You can use the focal length calculator in the build tools section. Use a 15" LCD and then you can test the available triplet sizes (320 & 500mm FL) and decide on split/unsplit to give the throw/screen size combination that you would like.

Try it out to see if you find something you like it terms of the layout.

cheers,
gs

Be aware that with a screen size that small, assuming the standard LL design, the projector is very likely to be right smack in the middle of your field of vision. There's a 1:1 ratio for screen size to distance from screen, so a 4 foot screen means your projector will be 4 feet from the screen.

Personally, I would not use it with anything smaller than about 65 inches or so.

He could always hang it from the ceiling, and use keystone correction? It might be out of his FOV enough.
I also saw a projector that was mounted on the floor shooting up at the screen on the wall. Seemed to work fine, just don't sit on it, or in front of it

what about a vertical design to sit on the floor like an end table? i have seen other plans for the vertical design but not alot of reviews on whether or not there is a defference in quality between the two.

whether its vertical or horizontal , the quality should be the same.... my first pj , I projected a 60" diagonal, occasionally it was annoying having the pj so close to the screen, but it really depends on where you sit(how far from the screen, and if you use keystone correction it should work fine....

With an image size that small, the angle he would have to project at would be MUCH greater than the typical 15 degrees, so focusing the image uniformly would be nearly impossible due to the severe keystone correction. He could do it, but it would likely be pretty ugly.

don't forget, you can also angle the screen to compensate and lessen the keystone correction.....
of course you could try to find a material to rear project on also.....

i had someone mention the rear projection. i am wondering how that would work? it sounds very interesting. also with the vertical design do you have to worry about the lcd sagging with 15-19" lcd over time?