You may not have to increase the largest size very much--ribbing stretches A LOT.
However, with that said, you'll need to add stitches in multiples of 6 (since the ribbing is 3x3). As for the ear flaps, you might even be able to keep these the same size as the original pattern.
I would do a swatch (in the round and in 3x3 ribbing, of course) to find out how many stitches per 4 inches you get. From there, you can figure out how many multiples of 6 you'll need to add to get the size hat you want.