There is no "formula" for sump size other than bigger the better as that gives you more water volume thus changes/issues in water chemistry,etc.. are not nearly as big of a deal..
But that also means more water, more cost, greater potential maintenance, possibility for more water on the floor,etc...
In general plan it out based on the equipment you intend to keep in there and go from that..
You don't even need a sump to BTW.. But its sure nice to stick a skimmer,etc.. down there vs hanging off the tank where you can see it..
But.. you don't even need a skimmer... so...
Its all about what YOU want to do with the tank..