I think you should shed a little more light on the subject at hand.
I know that there are minimum flange thicknesses and obviously if you're interested in the top flange, it should really be minimum since you've made the section composite and have necessarily moved the neutral axis up toward the top flange. So in the that region of composite behavior, its not economical to have a large flange.
The bottom flange is a different animal. There could be a number of reasons for having a thicker than necessary flange on the bottom. For example it is usually cheaper from a fabrication point to use the same thicknesses and only vary the width. This allows the fabricator to use same thickness plate stock cutting only widths. Otherwise the fabricator has to handle different plates etc. So in some cases it may be advantageous to use same thickness plates in composite and non-composite areas. Still, I am not aware of a max limit.
