I have a 61 cm 2017 P3 that I just fit up with Di2 this past spring. I have the battery in the seat post and an internal junction B above the bottom bracket which accepts the FD (300mm), RD (650mm) battery (250mm) and a long 1200 that runs up to the external junction A that I mounted at the front of the stem with a K-edge bracket. I have a wireless unit but haven't put it in yet as I also want to change out the external junction A to an internal to clean up the front a bit so everything is completely hidden but I haven't had the time. I could have gone shorter on the 1200 but I bought as a set and that's what came with it and it's easy enough to hide in the frame cavity like someone said.