This is what I have:

Sigma TS-31 Tripod Collar - for 120-400mm, 150-500mm and 50-500mm f4.5-6.3 Lenses

Product Dimensions 0.39 x 4.72 x 7.09 inches

Question:Does anyone know the inner circumference in inches of the collar? I'd like to use it on a different lens.
Answer:3 3/8 inch expanded, 3 5/16 compressed (approximately, it is not circular)

I don't have a metric way to measure it. I think I used a piece of gaffer tape as a liner to make it nice and tight. I sold the lens after a few weeks and the collar is sitting in a drawer.