newbie piping questions - what size return - what size overflow?

[Skimmerking]

IME
The days of having huge water volumes through a sump are pretty much over. Lots of people are converting over to having less flow in the sump for 3 important reasons.
1 to clean from the skimmer
2 to heat from the heaters
3 less chance of having micro bubbles
lots of people are going with the bigger water movers in the main display IE
Tunze's
Vortech's
closed loop
but for a rule of thumb the measurements are like this as the following
3/4"- 750 gph
1"-1000 gph
1.5"- 1500 gph
2"- 2000 gph
that is a approx measurement. But personally I would stay in the 1.5"-2" for the drainage just so that if anything goes down you don't get a clog from it.