I have a nice set of plans for a bat house, but my wife has hidden them. I don't have time to tear the house apart, so I will tell you where I got them--the public library. You pay for it, so you may as well use it. I checked out a book titled something like "Bird Houses for North American Birds" or somesuch and got plans for bluebird, wood ducks, woodpeckers, bats, etc. There was also great info on details such as best locations, heights, predator guards, etc. It was an excellent read, as well as providing the plans. It would be worth a trip to the library. Bats are very, very worthwhile to have around. Their natural roosting places are all but gone, so they direly need help.
Edited: I didn't realize my wife was reading over my shoulder, so now that my daily beating is over, she went and got the copy. I have several, so if you don't wish to get the book and just want the plans (with dimensions, etc), e-mail me your address and I will mail it tomorrow. I do not have a scanner, so can't do it via e-mail. [stick] OUCH!!