I'm in the process of getting a lighting USB cable as well as an HDMI cable that I'm going to have somebody else install for my unit. Can somebody tell me how long of a cord I would need for the USB and HDMI? I was thinking of just getting the standard 3 foot cord for each.

Depends on the relative locations of your head unit and where you want to connect your USB and/or HDMI devices.  Yes there is a limit to length... but typical vehicle somewhere-up-front applications shouldn't get anywhere close to that limitation.  If you get 6' cables, you can always hide any excess behind the dashboard.  If 3' is all you'll ever need, then get 3' cables.