Knot size is probably the biggest factor. That's the diameter of the knot. There are tiny brushes but I don't know why, maybe for mustache care only. My knots are all between 3/4 and 1"; that seems to be the standard size.

If you're cost-conscious you'll probably avoid the super premium 3-banded silvertip knots.

You said earlier you wanted a longer handle. That's why I recommended the travel brush--it'll give you that, plus it's designed to travel should you decide to break it down and stow it in a bag. Here's a link to some travel brushes. As I said, I got the black aluminum Muhle but wish I'd gotten the nickel-plated brass, as much for the mass as for the looks.

Not all travel brushes will give you that length. Some are, in fact, very short-handled brushes in a matching case. Watch for the ones where the case becomes part of the handle. Dovo makes one similar to the Muhle, a bit nicer probably and a bit more expensive too.

If your town has a store that sells fancy soaps and stuff, or a dedicated knife store, they might have brushes off the shelf, too.

Good luck with your shopping.