In reply to TobyA: The rim will usually have the size marked on it somewhere - probably 622 and a smaller number (622 means 700c - confusingly). If it doesn't, try looking up the model on the manufacturer's web site.
Sheldon Brown says that ISO/ETRTO rim sizing is measured between the inner faces of the rim:
http://sheldonbrown.com/tyre-sizing.html The tyre width does not have to be the *same* as the rim width, but the rim width does determine the maximum width of tyre that should ideally be fitted. That Sheldon Brown article, plus a bit pf reaseach on the rim and tyre manufacturer's web sites should tell you what you need.
Out of interest, can you share the link to the wheel you were looking at, since I might soon find myself in the market for something similar?