Depends what you're trying to achieve.
If you're trying to increase the stability of the system by increasing the water volume, then as big as you can get. If you're trying to fit specific compartments for specific reasons then that may dictate the size/shape of what you do.
I've seen sump tanks that are several times bigger than the display aquarium because of the space available, so there's no real limit.
The return pump needs to be less powerful then whatever overflow system you're using. After that it depends again on what you're doing with the sump and if you have any other circulation pumps.