Oops! I made a mistake in the pattern. I just wanted to let you know that I made a correction to the pattern. The pdf now include the correction. (The error was on the number of stitches for the petals.)
I have had a lot of questions about the hat, one of them being how to adapt the hat for an adult. This is what I would do. I would just increase the number of stitches on each petal until the hat reaches a circumference to fit your head. You would then knit the hat slightly longer, the decreases will be very much identical.