Matt covered it pretty well.
You don't need tons of flow through the sump, but try filling a tank and wish later you had put in large diameter bulkheads. A 1.5 inch bulhead can handle all the water flow even with a turbo snail crawling in it. Yes you can put screen and numerous other things to keep critters out, but just let that tank overflow all over the house one time and see what you think of the screen covered with caulerpa, a fish, snail, slug, or????? Oh, and while your cleaning up the spill ask you wife what she thinks....LOL.... JK, if you value your life.... I use a 1.5 inch overflow, a mag 9 return, and tunze 6060 for flow on my 90 gallon corner tank. The acroporas have to hold onto the rocks with both hands. Just 2 pumps run the whole tank.....