72x20x20tank?
 

Does ayone know how many gallons this tank would be?What size sump would be sufficent?Also one overflow or 2?It is going to be a coral tank.

tank
 

lxwxh divided by 231

125G.
The bigger the sump the better IMO.
Overflows depends on return pump size.

say if i only wanted 1 overflow. You can just get a bigger return pump?40gallon sump would be Ok?

To get an accurate volume with the above equation, measure the inside of the tank.