First, you need to know the thickness of the wing section at the spar location (chordwise). Your wing section will determine the maximum tube diameter, but the spanwise extension of the tube into the wings will also limit your tube diameter. Easy answer, go with the largest that you can fit in the wing. Your tube will have to be just a little bit aft of the main spar. I usually do some quick hand calcs, I really don't know of any online calculators. There are a lot of parameters that are used to size structural members. I have made a spreadsheet for calculating main spar size and wing tube size for my own design projects if you're interested in using that. I can shot it to you. Let me know.

Paul