My room is very close to that size and I run the exact same pump.
I went with 3/4" for my system. When trying to maintain pressure, a smaller volume is preferred. When trying to deliver the most mass, a larger volume is preferred. This particular pump performs more based on delivering a volume of air rather than high pressure, which is why I went for 3/4" PVC in a loop around the room. I wanted lower resistance and greater equalization of the pressure (which is also a big reason for using a loop instead of a single run). The extra volume due to the larger PVC probably adds a few tenths of a second between starting the pump and air coming out my sponges, but once the system is pressurized, there isn't really any negative side effects otherwise.
