Donk: No one can answer this question for sure, since it varies greatly from person to person. It is possible that you will grow some more, it is also very possible (I would say probable) that you are finished or almost finished growing. I think it is rare for penis growth to be complete by age 14, but it is even more rare for a penis to be much over 11" long. I would also say it is highly unusual for any penis to double in size in less than a year, but I guess it is possible. In my own case, I added the last 3" of my final length in about 6 months (when I was 16-17 yo). As for underwear, try boxers.