This was supposed to look like a fishing hat. I used the tutorial found here, but without any measurements or formula it was basically up to chance. The kid I'm making it for was reported to have a 21 1/2 inch head circumference, and I have a 22 inch cir. So it will probably fit, but ended up more like a top hat than a floppy hat. Another person on the wardrobe refashion challenge made a similar hat (here), but did some calculations and it's more like a proper floppy hat - so if you're going for the fishing hat, follow her instructions.
My top hat version was made from tracing bowls. The inside was 24 inches (circum), the tall part was 4 1/4 inches, and the outside circum was 38. I'd make a wider inner circum so it's less top hatty and more floppy plus a wider outer circum to give a bigger brim.