I hate to do this again, but ladies, this is what we call "knitting math". You can decide how much bigger you want the finished hat to be. 50% bigger, or 150% bigger. Once you choose a percentage, all the increases and decreases must also change by that percentage.
For example. If you have a pattern that says cast on 100 stitches, and you want to make it 10% larger, you would figure 10% of 100 and add that many stitches to your cast on.