What are you planning to do with the sump, since it is a freshwater setup? For FW
, it can be as big or small as you like, depending on what you want to accomplish... obviously the bigger the volume of the sump, the more stable your water will inherently be in the whole system, so imo
go as big as you can fit.
For a DIY overflow box, just do a google search for DIY Overflow. There are lots and lots of plans, threads, and videos out there about it. Alternatively, you could just purchase a premade overflow box if you don't feel comfortable trying to build one.