it depends on several variables.But as initial estimation one can use deflection limitation then revise if design results pointed such need.

A typical I section for short span bridge->

The plate thickness is ¾” for compression flange(width 12''), 1 3/16” for

the tension flange(width 16") and ½”for the web(height 41").

please see more ->( www.aisc.org/globalassets/modern-steel/archives/. )

Depends on the load on it, could be short but for railway syste. So if you referring to road bridges also depends on the spacing between girders/width of the bridge. I would say W24 and larger.