Depends on how you design it and what you want from it.
For a single hoop, provided it's not too tall, you should be ok with 1.5", but personally, I'd go for 2", with 1.6mm or 2.0 mm wall. The problem with 2", is that less people are able to bend the tubing, whereas 1.5" tends to be no problem. You can buy pre-formed bends and weld these to straight tube sections if this is a problem. Don't even think about towing off a single hoop A-frame.
For a twin hoop design, 1.5" OD, 1.6mm wall would be fine, but if you want to tow a skier/wakeboarder off it, then, you'll need 2"OD, 2mm wall, with the forward hoop fixed to the deck/hull.
Would be good to see your sketches.