I base wheel size on tire selection:

I wanted 225/45/17's This is a typical BMW size, however this was too small for my OE diameter of 195/65/15, but this would be an excellent 5th gen size.

Now some VW's, Bimmers, and Lexi (Europe) use 225/40/18, comparison to 195/65/15 yeilded a good match. So I went w/ 18'

I wanted to be sure I had an OE sidewall spec, tires are cheaper this way and you know the sidewall can take a trip down the road with a secure feeling that somewhere a car of similiar weight to yours is using this type of tire stock.