HD, you'll probably need 2 'Y" extensions. One for the ailerons which will be plugged into one channel and another for the flaps which will be plugged into another channel. As far as size, get one just large enough to join them at the wing, I'm assuming a 6in would work fine. Then just depending on the length you would need to be able to connect the extension running from the rx to the "Y" extensions, I would again a 6in servo extension would work.