The Terex is ~2.28 times the capacity of the haul truck, so the size of the in game model should only be scaled up to 1.32 to 1.4 times the dimensions of the haul truck. (Volume increases by cube so 1.32^3 is ~2.3 times the volume)
The fuel use nearly 2 times the dump truck, so the fuel tank should probably be double the capacity of the haul truck.