I think it totally depends on your camera. I have used a couple different sizes. I don’t think the standard Freefly tube is relevant with the brushless gimbals you and I are using.
Hi Steve I'm rebuilding my 3 axis gimbal that I want to use exclusively for the 7D to take stills (using radians). The 3 axis gimbal was in my crash. I forgot to measure the tube before throwing it away.
Well, mine is 115mm, but I think I’m using different hardware (boom connectors and roll cage) than you are, so YMMV.
I think the 3 axis is 125 mm default, mine is now 150 mm for better boom clearense with wider lenses. Regards, /Janne