I have a hard time figuring out when cinnamon rolls have doubled in size. Any tips? Thanks!

I eyeball it or poke as Stephenie suggested, myself. But you could take something like a chopstick, mark the height of the rolls before rising, then put another mark at twice that height.

I like to give any yeasted bread a poke to see if it has risen enough. If it springs back just slightly but holds the indent, it has risen enough.