It depends on the size of your skimmer... you need to have room for the skimmer, so it needs to be at least that big. The more LR
you have the better filtration you have. With building the sump it is kinda up to you... as long as all the equipment fits that's all that matters.
If you have a sump you don't need a canister or a hang on the back filter.
30g is better then 20g but 60g would be even better, again there is no rule, it's just more water volume is better (no limit on that).