I really had a struggle with figuring out what size the sump needed to be to handle the water load from my two grow beds. As LUCK would have it, the system as built works perfectly. With the media in the growbeds the amount of water draining back to the sump tank is about 3 or 4 inches under the rim. Perfectly sized. To that end, let me share my bed sizes and capacities...
Note: 0.00432900433 is the cubic inch to gallon conversion factor
IBC tank with the top cut off.
36 x 36 x 44 = 57024 cubic inches
57024 x 0.00432900433 = 247 gallons
Grow Bed #135.5 x 72 x 12= 30672
30672 x 0.00432900433 = 132 gallons
Grow Bed #2
38 x 72 x 12= 32832 cubic inches
32832 x 0.00432900433 = 142 gallons
Total Growbed Gallons: 274
FT/GB Ratio: 1:1.11
GB/ST Ration: 2:1
In conclusion, it seems that the sump size should be half the size of your growbed capacity. Total growbed gallons / 2 = sump capacity.