My take on this is somewhat different. 1st, figure out what your water flow needs to be. A 1" pipe can only drain about 800gph for example, so using a pump larger than 800gph would flood your tank. Once you know your flow rate and therfore your ideal pipe size, you can choose the type of pipe. As danny zubot wrote, larger piping is louder, so be a minimalist at this step.
Flex line is easier to work with but the water does not drain as well as through a rigid pipe. Think of how the water moves through a bending waterslide, it sloshes, you don't want sloshing. I use flex in tight areas (like in my overflow) but rigid everywhere else.
For sump size, go to a store that sells your brand of skimmer, pump, heater ect. and place all of them into the sump for an idea of space consumption. Most of us will add more toys as our tanks mature so leave yourself some extra space. Newguy is right in putting in the largest sump possible. It will make life easier, just think, you won't have to top off water as often!
Hope this helps
Rick
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