Lens boards I've seen for 8x10 and larger cameras are anywhere from 3/8" to 3/4" thick. Generally they tend to be thicker the larger they are, which makes sense. As Ian pointed out, if your lens uses a mounting flange rather than a retaining ring, the board thickness becomes all but irrelevant. And as Nick suggested, the mounting hole can be counterbored in the back to allow using a retaining ring. I've seen this especially for boards intended for a Copal 3 shutter which uses a retaining ring.
eric said:
Also, in general, are the lensboards anywhere between 6x6 up to 9x9?
Generally, yes. Wisner uses 5 1/4" (nominal) lens boards. I think Canham uses 5 1/2" boards, but I'm not certain. I think that's also the Sinar size. Deardorff, Burke & James and some Agfa/Ansco cameras use 6x6. Other Agfa's use 7 1/2" boards. Larger cameras, esp studio types, use 8" (Deardorff, with rounded corners) or 9" (Agfa, B&J). There are a few odd sizes larger than that, but those sizes are typical of what you'll encounter.