In general, the return line should be the same size as the supply line.
On Carb (lower fuel pressures) you need very low restriction back to the tank (supply line size or larger) because that will be the lowest pressure you can adjust the regulator for (when the pump is full output, and no fuel going to the carb float bowls.
An Example would be when my '71 Charger was running the Mallory 250 pump (full flow) with 1/2" supply and return lines. I think that pump puts out around 20 psi un-regulated?
It was difficult to set the By-pass regulator pressure (at the engine) at idle because it was by-passing so much fuel the lowest pressure I could get was 7-8 Psi. Because the pump has a built in bypass regulator, I used it to bring the pumps output pressure down to maybe 12-15psi, then I could adjust the by-pass regulator at the carb for 6-7 PSI

On EFI (high fuel pressure) you can usually be OK with a return line one size smaller than the supply line. With EFI pressures usually around 3 to 4 bar (43.5 to 58+ psi) a little pressure in the return line is OK.
For example, if the restriction in the return line is 10 to 20 psi that is still much lower than the EFI regulated pressures of 40+ psi, but would be way too high for a carb requiring 5-7 psi.