The kind with extra holes all the way to the end will be fastest, but if you choose a different kind, all you have to do is add more holes where they need to be.
Most belts have a relatively standard width and buckle size that's more appropriate for medium or large dogs, but there are thinner ones out there that would work for the little ones.
You'll want to make sure the one you choose doesn't have a buckle with sharp edges, and that it's not bulky enough to cause any discomfort.
The metal loop that the end of the belt tucks into should be thin enough to accommodate the snap hook of your leash, but strong enough that it won't come loose if your dog pulls. If it's too snug for free movement, you should be able to get something (like a small carabiner) at a hardware store for a dollar.
Sharpie, marker or pen
To show where you need to cut.
For cutting it to size.
Leatherman or pair of pliers (optional)
For reattaching the tip. Not necessary if you choose a different kind of belt, or like it better without anything at the end.