I never think of myself as...

Torvid

5'11 - 76kg (12st)
33" waist
38" chest

Medium in most brands but a large in race cut italian tops.

Ben6899

5'11" or 180cm
11st or 70kg
38" chest
31" waist

The jersey that fits me best (so I have a couple) is Mavic's Espoir (http://www.mavic.com/en/product/apparel/jerseys/apparel/Espoir-Jersey) in medium. Most other stuff is medium as well although the fit is not a patch on the Mavic stuff for me.

At the end of the day, two blokes weighing the same can look completely different.