All you wanted to know about propane--and more:

http://jones.colorado.edu/VW/PropaneFacts.html

Nominal capacity of the original Westy tank is 2.5 gal
which converts to about 11 lbs.  When I have mine filled
it typically takes 2.5-2.6 gal.  The meters on the
propane fillers are only approximate anyway.

I believe some newer replacement tanks are slightly larger.

Richard