19.5mm inside and 22 mm outside. But the mount ends are impossible to remove so you can't fix a broken arm with a cheap tube from a third party source if that's what you're thinking. I thought of that too except I wad going to get thicker ones to make it stronger so they don't break so easy. Thin cheap crap!
It is not impossible to remove, but a real pain indeed.
What you need to do is dremel tool the carbon tube on the inside so it weakens. Then you need to release/break the tube from the plastic pieces, like with a screwdriver or pliers. When you jam the screwdriver in there and the plastic flexes away from the tube the epoxy will crack and release. the plastic bits are quite resilient to bending. The middle metal piece I popped in the oven on HIGH and wait for it to make loose then grab the end with pliers and pull it out.