go with a large or extra large, I have a large and I have filled it up basically to 100% and its not to comfortable to carry that way I was also carrying shoe box, a hard case about the 1.5 times larger than a shoe box, a couple of text books, a jacket, a ha,t a lock, a nalgen, a couple of 1" binders and some other assorted crap
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Rams Cycling Team
'06 Cervelo Soloist Team record, '04 S-works epic sram and king equpied, '03 litespeed hyperion DA/Ultegra 10sp