In a fuel injected engine such as the 13B you need
enough for the engine plus the fuel
that is returned to the tank.
Also unless aggressively leaned the rotary will burn a
bit more fuel for the power delivered so that will also increase fuel flow
requirements a little.
I assume we are talking about the fuel
tank vent. No need for vent air for fuel that is returned to the tank; only
for net outflow. Anything much bigger than a pinhole will allow enough airflow
to compensate for fuel flow out since the viscosity of air is many times less
than fuel. But it doesn’t hurt to have a bit more, and use a readily
handy ¼” aluminum tube. I have about 3’ of ¼” aluminum vent
line, and I know that it will allow more than 30 GPH. That was tested with
gravity feed of fuel through a 3/8” line, and I’m quite certain
that the limiting factor was the fuel through the 3/8” line.
FWIW,
Al G