If you mean the painting surface go to the site below and choose Beams -> Sections -> I beams , get the beam type and standard you are looking for, then recalculate and you are done.
There is also a table in AISC's Manual of Steel Construction - Allowable Stress Design, 9th Edition and in the LRFD, 2nd Edition, which will give you the surface area per foot of various structural shapes, or you can easily calculate it by hand.
