Hi tim,
As told by jae if u could specify the loads on roof at site(as per local code) it would be much simpler to determine the beam size.
Also specify is the structure a wooden or of any other material.
If wooden then the post supporting steel beam need to be steel post thus some connections...
UBC'97 Table 23-II-I-1 has a footnote 1 which says
"multiply the Struc-1 values by a factor" .82 or .65
Example: T-1-11 siding applied 8d 6"oc edges
SG=.50 for DF framing: 160 plf (from the table)...