A fuel system is only as big as it's smallest component. Don't forget your vent(s) and other (e.g. tank fittings.) Also, the fuel line size also matters. If the fuel pump is in the turbine, then everything needs to go up in size. As Dave suggests, 90 size is a good breaking point for a turbine with its pump in the turbine. My Ultra Flash's P-140 had a problem with a small UAT even with larger vent(s) and lines. It has a main tank and the saddle tanks.
The size of all fittings and UAT, etc. is also dependent on what your tank arrangement is. On my P-140 RXI, the turbine was not able to get enough fuel flow through the main tanks and two saddle tanks. After changing to a High-Flow UAT, it still had a problem because every fitting in the system, including the vents, tank fittings, etc.had to be high flow too. Of course bigger fittings = bigger fuel line size too. Once everything was set with high flow components, then the P-140 RXI worked correctly. Oh, I also ended up with a high flow vent on each of the saddle tanks and they were in parallel versus series.
The above being said, my K-160 with an external fuel pump, works just fine on a 4mm line between the pump and turbine. But all the fittings and lines are high flow on the inlet side of the pump. I use the smoke tank for extra fuel and the system works fine drawing through the two tanks.