It all depends on the running level of the water in the sump. I.e. water level required for the skimmer etc, PLUS whatever amount of overflow you will have when the return pump is off. A little bit of quick math should help..
For reference, to find gallons of water, use this formula.
*All dims are in inches
Length x width x height ÷ 231 = gallons
You need to figure the gallons of running height water in your sump + gallons draining from the display when the return pump is off to see if a 30 gallon sump will be sufficient.
That said, bigger sump is ALWAYS better. More room for whatever ends up being in there...