Assuming this is a "Do it yourself" site using as little money as possible and not relying on someone else to do it for you, I have a procedure for you to stretch a felt hat to fit your head.

Start boiling some water in a sauce pan or skillet. Once the steam is rolling, hold the hat by the crown over the steam.

Once the felt becomes soft enough to work, Take it away from the steam and as soon as you can without scalding you, Cram it down on your head as far as it will go and let it cool completely.

Repeat the procedure until it won't stretch any more, or until it fits. If it needs to stretch some more, Start putting sheets of newspaper or similar material "BEHIND" the sweat band and repeat the procedure.

If you keep at it, it will stretch to a custom fit.

I have done it MANY , MANY times on my own hats that have been used and abused.

One thing that shrinks them up is heat. Leaving them in a hot truck is really hard on them.

Once it fits, get a large piece of solid styrofoam and cut it to fit inside the hat for storage. Make sure you put a couple holes in it or a handle on the styrofoam so you can get it back out.

Works great.

Muleskinner

Thanks! I'll try this when I get home.

if it were me, i'd just get a new hat. even if you get it to fit, a wool felt hat will shrink back down pretty quick. i'd recommend checking out Akubra; i've got a Federation and i love it to death. and with the current exchange rate most of them come to just a bit more than a professional reblock.
http://www.hatsdirect.com/cgi-bin/products.cgi?group=Akubra+Hats&page=1