To start with AN fittings are measured in 16ths. A #1 would be 1/16th, a #2 would be 2/16ths or 1/8. A ½ inch line would be #8.
It is true that most pumps come with a 3/8 in and outlet but they pump the advertised volume.
What most people do is run a #8 from the tank to the pump, this keeps the pump from starving for gas, that run a #6 or an #8 from the pump to the carb or regulator.