When you find just the right, hat but it is too large, try resizing it. This is a guide about resizing a hat.
Here is a great way to make a large hat smaller. Attach a piece of elastic between two safety pins and pin towards the back of the hat. The hat will fit snugly against your head and no gap will be noticeable.
By Stella Bella from Manchester, WA
Another method I've used for stage work - take a piece of plain cotton or muslin about 12 inches wide and a little longer than the circumference of the hat, fold and stuff the band of material into the hatband, adjusting until the fit is rignt. Most of the time in men's hats there will be a pretty generous hatband, and it won't need securing. If you need to secure it, just use a little duct tape to hold it in place.