What size of axles do you guys recommend?
I'm assuming you are making your own axles? You'll want something fairly thick in the middle. I would say probably 10-12mm thick in the middle. I have no idea what kind of constraints you are under though. I'm estimating off the size of my Baja 5B axles. I'm assuming you want something similar in thickness.
Some say that he urinates 98 octane gasoline and that he sees everything in binary...
Make it idiot proof and someone will make a better idiot.