Let electronics do the work.
Once you have your Di2 switches, be it road or time trial dual control and satellite switches, you'll need to finish the job with two Shimano Dura-Ace 7970 Di2 Internal Wire Kits -- the Shimano EW-7970 for the rear (lower) harness and SM-EW79A for the front (upper) harness. The Di2 Rear Wire Kit, EW-7970, has long enough wires to reach the derailleurs and the battery pack. It plugs into the five-pin connector below 'Junction A' on the front harness, which plugs into the shifters.
For the rear harness, there are three different harness lengths. The smallest will fit on bikes from 36cm and larger. And the largest will fit bikes up to 66cm. Shimano tells us that the Medium kit will fit most bikes -- being that there's a cable wind on 'Junction B' that is positioned below the bottom bracket where you can wrap up to 120mm of cable slack. The Medium seems to fit bikes up to 58cm in size, though that should only be taken as a rough guide.
To determine the best sized external harness for your bike, Shimano recommends measuring from the bottom of the bottom bracket shell to the back of the bottom of the head tube. Call this measurement 'B.' Then measure from that same point on the bottom of the head tube to the bottom of the handlebar next to the stem. Call this measurement 'A.' Add A and B, and subtract 30 to get X -- the size needed. The Small is 770mm long and handles measurements from 650-770mm. The Medium is 830mm long and can handle measurements from 710-830mm. The Large measures 890mm in length and can take care of measurements from 770-890mm. If you have a bike that's designed to have an internal wire through the down tube, and have the battery mounted under the bottom bracket, select the Short Cradle version. Likewise, if you're bike frame is Di2 specific and built for the wires to run completely inside the frame, select the Internal version. Both the Short Cradle and Internal harnesses will accommodate the largest frame sizes.
When you look at each kit, it seems like there are four wires. That's due to Junction B. Essentially, there's a wire from the front harness to Junction B, wires from Junction B to the front and rear derailleurs, and a wire from Junction B to the battery. In addition to the wiring harness, the kit comes with the battery bracket, six segments of Black cable cover to secure the cables to the frame, two different length screws for securing Junction B to the bottom bracket cable guide on the frame, and a cable tie which can be used to further secure the battery bracket to the frame.
The Shimano Dura-Ace 7970 Di2 Internal Wire Kit is available in the color Black and in the options for EW-7975-A2 Internal, EW-7975 Internal, and EW-7975-A1 Internal.