[mr_alberta]

I personally would use the largest sump that I could physically fit into the area I have. A little more water volume never hurt right?

As for the type of piping, I have worked with both rigid and flexable pvc and I'd have to say the flexible pvc is much easier to work with since you can bend it and whatnot. You might also reduce headloss with flexible pvc since you might not have to use as many PVC elbows and 45's.