Hello there, and thank you for reaching out to us. The arm itself, end to end, is 37.38". It'll open up to a 90 degree opening.

Thank you for reaching out. The secondary unit (DCS) comes with a master/secondary cable to connect the farthest arm to the control board. Each arm comes with a short cable but when you have 2 arms, one arm will be situated farther from the control box, which means you have to extend the wire, hence the included long cable for the secondary arm. Liftmaster has a youtube channel you can look through but they don't feature the LA400 rather the LA500 model which works the same as far as wiring goes