square tube doesn't have a diameter. if you are talking about width/depth to an equivalent diameter, you can just look up the catalogues/tables on websites like onesteel. they provide all the data. "I" for deflection "Z" for bending. higher number = winning.
It's not in bending anyway it's related to area and I value.
square will look shit, if you're talking about strut braces... which I am assuming you are?