New to Typophile? Accounts are free, and easy to set up.
I was visiting http://www.letterpress.ch and while some information pages are no longer valid, the .PDF downloads are still there.
In reviewing the unit arrangements for a few of the well-known typefaces, I found out that in Poliphilus, the capital letter W was 21 units wide, which is more than 18 units. In fact, looking at the unit arrangements document, I see that several other typefaces go over 18 units; several have characters that are 20 units wide, and a few go up to 25, 28, or even 29 units.