darin,
A quick reference for finding the filter size on most any lens is the listings @ KEH Camera.
The filter ring size is identified with parenthesis '( )' for most lenses they list. Very handy!

Random KEH listing example for a Canon lens using 48mm filter:

Canon Manual Focus - Fixed Focal Length Lenses
135 F3.5 FL BL (48) 35MM SLR MANUAL FOCUS TELEPHOTO LENS

KEH Camera, Georgia, USA:
http://www.keh.com/

Marc