The rear axle size, to go through the chain adjuster block holes, is 14mm.
I used a front rim for the rear as this was the only way I could get a 3" rim on the back with a smaller axle size (in this case an FZR400 rim). Making the spacers for the correct width was another mission, as was converting to rear disc. You make the bracket hanger for the rear caliper off the RHS axle spacer.
A problem I have is that it's all quite a tight fit with the slick on there and the chain just touches the side of the tyre.

The front rim is a 2.15" TZR250 rim. That axle size (10mm) fitted perfectly, but once again, a really tight squeeze with a little shaving off the bottom of the forks (I didn't like doing). I had a stock of wheel spacers so it was just matter of coming up with the right combo, LHS & RHS.
The other issue I had was I had to swap my forks over and spin the RHS fork around as the TZR wheel is a RHS disc. Then it was fabricate a caliper hanger.
SHEEEZ.... I'm getting tired just thinking of all this................

Come up to my place some time before the next meeting and see how I've done it.