The amount of water your sump can contain is determined by your tank size and the depth of your overflow, so since I dunno the size of your tank I can't tell you that, lol.
Here's a great write up on sumps in general (especially for reefs/SW though)
http://www.reefkeeping.com/issues/2003-01/gt/index.php
And a calculator to show how much water your sump needs to be able to handle in the case of a power or pump failure.
http://reefcentral.com/calc/sump.php
Just enter the length and width of your tank (main tank, not the sump tank) and it will tell you how much space you need to leave in your sump.
Here's a great write up on sumps in general (especially for reefs/SW though)
http://www.reefkeeping.com/issues/2003-01/gt/index.php
And a calculator to show how much water your sump needs to be able to handle in the case of a power or pump failure.
http://reefcentral.com/calc/sump.php
Just enter the length and width of your tank (main tank, not the sump tank) and it will tell you how much space you need to leave in your sump.
