I bought a 75 gallon AGA tank and used that for a sump (48 * 18 * 20) on my 210 AGA.
My tank drains about 15 gallons and the sump holds it OK. BTW, I'm also using a refugium in there so there's plenty of space.
So to answer your question. There's a calculator here (RC's main page) that will help you calculate the sump volume needed.
I took the liberty and did the calculation for you assuming you'll use a 1200 gph pump and the math comes out to this:
Using the following input parameters
Tank Length = 72 inches
Tank Width = 24 inches
Height of Water above Overflow = 2 inches
Flow through sump = 1200 gph
Sump volume is calculated as
Sump Volume need to handle tank overflow = 15 Gallons
Recommended Minimum Volume for sump = 27 Gallons
According to this you'll need around 15 gallons of free space for your overflow.
In a 48 x 18 sump that equals to a little over 4 inches of water.
Here's the math:
(48 x 18 x 4) / 231 = 14.96
Of course, everything changes with different parameters
HTH,
djfrankie