Is anyone able to tell me what length EW-SD50 Di2 wires I need to get for a 54cm AR frame?
2 x 350 ST
1 x 400 FD
1 x 450 RD
1 x 1000 (for ease of seatpost adjustment/removal)
1 x 750 DT
You can use a shorter wire in the seat tube and downtube if you want to shave a couple grams but it makes installation and service more challenging. For OEM I specify 550mm on the chainstay RD wire so when packing/traveling with the bike you can removed the R/D from the hanger and wrap/pad it and tuck it into the rear wheel so any external impact doesn't damage it as it isn't e..xposed that way.
As all wires will be routed internally except the handlebar wires (which can be tucked under your bar tape) there is no risk in getting the longest wires available and just stuffing the extra in the frame.
When you order you kit be sure to get SW-610 sprint shifters. These are a huge add-on worth the price of Di2 alone.
You might also consider a 5 port Junction-A if you ever want to add aerobars + shifters to your ride. I use the 5 port junction on the AR FRD OEM loom for that reason as the AR makes a handy multisport bike, too.