What is the actual diameter of our current bicycle chain's rollers?
I've measured three well-used chains with vernier calipers.  Below are the method and measurements.  I'm assuming you mean External diameter of the roller in the chain.

TL,DR 7.66mm +0.04mm -0.02mm measured.


KMC X10 10speed chain with 4223 km 7.64mm

Old Shimano CN-HG93 9 speed chain with 3800 km 7.70mm

8 speed folder with cheap Clark chain, around 2000 km  7.64mm





Method - my calipers were binding on the inner plates, so to get a good reading I had to make the chain slack and then pull up a roller

BAD 

GOOD 

I used an old spoke as a hook to take the tension off while taking that photo.