Depends on which side you are replacing. Drivers side is 73mm and is held on with a metal flange that bolts to the gearbox. The passenger side is 80mm and is held to the gearbox with a clip.
I always replace the bearing when I change gaiters, the bearing has to be removed to change the boot so it is a false economy not to fit a new one. Plus there is no worry of damaging the bearing when removing either.
You will need a suitable drift to reinstall, I have a custom made tool I would be happy to lend to you Ian.