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