With the large size of the basin and the small flow, the size of the outlet pipe is a small matter. I would make it large enough to be sure that it won't suck in air.
I would pick an outlet size so that the velocity head in the outlet does not exceed 10 percent of the depth of water above the outlet. The inlet to the outlet should be rounded to prevent formation of a vena contracta.
It is customary to make the inlet to the outlet pipe at least 50 percent larger than the inlet to the pump, but I would not make it less than the "10 percent velocity head" condition mentioned above.
I would also plan on using a screen to prevent trash from getting sucked into the pipe. That screen would contribute to preventing formation of a vortex that would suck air.
If you want to do an analysis of the problem, you can look up the analysis of a "free vortex" in an advanced reference on fluid mechanics.