Pretty sure the tray is 150mm long. I know for a fact that Turnigy 5000mAh 40C packs fit regardless of the cell count. I am using Turnigy 4S and 6S 5000mAh 40C and 5S 5000mAh 30C packs in my RC8Te and RC8Be. They fit although just. It's a tight squeeze but I have no problem getting them in and out of the battery trays.

My RC8Te runs a 1518 now but used to run 1520 as well. I had to use the 1520 in another project so I fitted the 1518 now. Anyway both motors have been ran on everything from 4S to 6S and no issues at all. Well I did have a few tires separate from the wheels.