I never measured it but Mike at TKO says that if you want to use the Disco iron sights that you can glue the disco front sight on top of the muzzle brake and it will be at the same height that it was at when it was screwed into the top of the discos factory muzzle weight,,,,, so that should give you some idea as the outside diameter.
The TKO brake, once installed, prevents you from using the discos fill cap unless you modify it. Mike at TKO will modify it for you for $10 including shipping. I just mailed him my fill cap.
Mike at TKO could easily tell you the outside diameter,,, send him an email..... he replies to emails extremely quickly.
Jeff