Your pump output size would determine your return line - 3/4" in this case. I don't see any advantage in using anything bigger since the 3/4" outlet at the pump would act as the bottle neck to flow anyway.
The drain back to the sump must be sized to safely handle your pump flow. An oversized drain will add some margin of safety. Also a Durso or Stockman attachment works better with bigger pipes. A 1" pipe should handle 600gph. If your pump puts out more than that, you need a bigger drain.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.