lol. you may want to try to find a local reefer with experience to help, but just get the white PVC, and a bottle of PVC glue. other than that, its just straight runs, and connecting fittings like 90 degree or 45 degree bends (always best to avoid 90 degree and do 45s when you can, but its not always an option) and what pump do you plan on using? IMO, its difficult to oversize your drain pipe so big that it causes an issue, but you can easily under size it.
with a 20gal tank, and 15gal sump... you are going to have say, maybe 25-27 gallons in the whole system, possibly as many as 29-30 of course, depending on how shallow your sand bed is, how much LR you have, what all is in your sump, etc etc. i'd plumb it with 1" or 1.25" ID, keep the flow fairly low to allow the water in the sump/fuge to have time to take advantage of the - whatever you decide on in the fuge, and the skimmer to work, and still create some return flow, but not too crazy, then add powerhead(s) to keep the water moving, and avoid any dead spots.
as for 30 gallons of water, the general rule of thumb would be that you need a 200ish GPH pump, plus a powerhead or two, to turn over the water 10x per hour.
however, this is before you factor in all the resistance from the plumbing, bends, valves, T's, and the like.
sorry...to make a long story short:
I'd get a 300gph pump, and 1" plumbing for return, and 1" or 1.25" for drain. throw in a powerhead, or two small powerheads, and keep the water moving. lol
is it a 20L? 20H? Standard 20?