The booth should be no smaller than 4x6, which is small. Imagine your arm-span being roughly 6 feet, and with two people in there, it's not bad, nor would it take up much space. Add a foot or two on either side and it gives you much more space, so try to mark it out and get a feel for it. But 8x6 would be really ideal for two people. I've been in 4x4 booths, but that was just TIGHT.
If you have the time and budget, you should take the floorboards out under the booth and make it have its own floor. That greatly reduces the vibrations from the control room, or anything else in the house.
As for the glass, you should bank it so it's not parallel with the walls, and it reflects upward toward your ceiling. On your ceiling, you can put deadening material so it juts out at an angle and catches what comes up from the glass. But you don't want the glass to reflect in the mic, so always put the mic off-axis from the glass.
Sound-absorptive material is real expensive, but if you coat the room with it, you're good. But you can always get a little at a time, and get the corners of the ceiling and angle it downward, and if you can, parts of the walls directly behind and in front of the mic. If not, there are always egg-cartons. They don't work perfectly, and if you paint them, then the paint becomes reflective, so it might look shitty. But when you do put up your deadening material, imagine how the sound travels and what it would reflect off of, and place it there. Every time you put another piece up, clap (you might want to even record it to compare reflection levels if you're crazy like me) and make noise to hear the room until you don't hear the room.
FIRST but not least... during construction of the walls, get a good snake with more ties than you need, and run it through the wall. When you make your input tie box, you might only have 2-3 XLR inputs, 2 1/4" inputs, and 2 1/4" returns for headphones or something, but at least you'll already have the cable in the wall, should you want to add more channels later. And (assuming you are making a input-tie box) buy a cable tester and test the impedance of each lead from the studio side to the other end of the cable after you solder. Check tip, ring, and sleeve on 1/4" leads, and check pins 1, 2 and 3 on XLR leads. You're not looking for particular numbers of ohmage, but just check for consistency. If you see a drop, or a large amount of impedance in one lead, re-solder it and check it again. So many people get anxious and don't even test their cables, and later, they get static and hiss and dropouts in their recording.
Hope this little bit helped.
pce.
