That's a very open question. I (not a plumber) am doing 2 of those now; one is a single vanity, double bowl, could be anywhere from 5' to 8' long. The other is actually 2 vanities across the room from each other, with the bowls offset so that the users don't bump butts. Both present different problems. The client has to decide some obvious stuff before the plumber can do his work, but sometimes the existing plumbing will dictate allowable configurations. As for height, it can also vary from very low (for a handicapped user), to very high (for taller folks), to adjustable (very fancy, with more complex plumbing problems). I set up a temporary single vanity for one client and we fiddled with the height for a few days, and she eventually decided she liked a 34" overall height. So one parameter is checked off. Supply plumbing is usually easy -- flexible lines provide a lot of ...um, flexibility. A good plumber can advise you wrt waste plumbing, taking into account venting issues, developed length of drain lines, etc., that are often not obvious. One caution -- if the waste line is directly behind a bowl, there may not be enough room for the trap in a narrow vanity situation.
