15/39 are the stock sizes. For what it's worth, on my 15 I recently did a 520 conversion. I bought it used in 2017 and have always had issues with my autoblipper. I assumed it was bone stock gearing since everything else was (had 1700 miles on it). Come to find out, the previous owner had put a 14 tooth front on the bike. Well, since I had ordered and received the stock front from Sprocket Center, 15, I mounted it and to my surprise it fixed my autoblipper issue! Since you're planning on going with stock sizes (I did too) then you shouldn't have any issues.