You are huge! Way above the average of a touch over 5" long and just under 5" circumference. You should be hoping that it won't get any larger at this point.I was wondering if this is below average, average, or possibly above average and if it could possibly get bigger, and if it can, then by how much?
Your chest is certainly big for a 15 year old. Arms are quite big but not huge. I can't tell from the photo you provided what your body is like and how much body fat you have. An overweight guy could easily have a big chest and big arms without it being muscle.