I got this amazing pattern and went right to work with the recommended size 9 needles and worsted weight yarn. (to fit 21" circ. head) I thought it would fit a teenager, but it was more like a toddler size! I guess I should've done a gauge swatch, but the pattern led me to believe that it was for s larger child. I'm going to try the bigger needles (10s) but maybe there's another pattern out there designed for teenagers?
Before you rip it out, measure it in a couple of places on the dark yarn and see how many sts per inch you got. Divide that into the sts you cast on to see how many inches that turns out to be. If it's not 18-20" it's not big enough and you can try the larger needles. It may be though that the sts in the stranded section were pulled too tight and that's what made it too small and you might be able to use the same needles. just keep the floats very loose on the WS.
Well, I didn't rip it out. I have a friend who knows a mom with a 4 year old for whom the beanie is perfect. So she was going to pass it on to her. I just have to try again. It is a terrific free pattern on polarknit.com.