You can just measure it when you get it

There is not much money to save, so people restore brushes for other reasons, i.e the handle is really cool, old, special or something. What you do is remove the bristle, as much as you can, with a pair of scissors or the like, then find a way to remove the rest. Many use a drill, a dremel, or even a chisel or a small bladed knife. Whatever you are comfortable working with. Just don't ruin the handle Then just have at it with all the restoring you think it needs.

Plug sizes - A 22mm bristle plug will not fit in a 20mm hole without fiddling with it. However, a 20mm bristle WILL fit in a 22mm hole.

Best of luck, and have fun.