There are a couple of things you can do when a fur felt doesn’t quite fit. If the felt is too small, we suggest taking it into a store that shapes hats. There, they can use a hat stretcher and stretch it between half a size up to one full size closer to your fit. Sometimes the hat may revert back to its old shape, with time.
If your hat is way too big, you can buy a bit of foam sizing tape to put under the sweat band. This will usually bring the hat down half a size to a whole size depending on how much sizing tape you use.