Question about Garmin Handlebar mount

As mentioned, the handlebar support is expandable. I have mounted this handlebar support on my MTB (handlebar diameter 2.4 cm) and on my road bike (handlebar diameter 3.2 cm). No problem.

I had the same question. I am also interested in whether the thing can be mounted on an aero handlebar?

It's made of rubber and can be stretched quite a bit. I can't tell you the exact circumference. How much circumference do you want?