Grab a piece of string, wrap around, mark where it meets, measure mark to mark then simple math. C=piĂ—D or D=cĂ·pi (where D is diameter, c is circumference and pi is 3.14159) and because it's metric, multiply that by 25.4 to get the mm measurement.
Or grab some calipers. If I knew where mine were I'd run out and check. I'm thinking roughly â…ž" diameter or 22ish mm.
They're split so if you're close enough they'll still work. Come to think of it, I might have some spare bushings, I'll check and let you know. If I recall there were split flanged and non-flanged tubes that came with the passenger backrest kit. I think the split went on first then the tubes so they'd spin freely. My memory isn't so good 10 years later.