might as well go 300 gallon
The corner overflow really only needs to be big enough to fit plumbing, flow depends on the slot size, and on a 10 I would want a small overflow with bigger slots to take up less tank space. The hard part is going to be matching up the volume of the slots to meet the flow needed. Other than trial and error, I don't know what to tell you there.
Bio speaking, if you don't have a sand bed to move I wouldn't worry about it, just keep the buckets heated and airated