I think he meant he has a 125 gallon DT, what size fuge should he have...
If that's true, on a 125, I'd shoot for 15-20 gallons. That's bigger than the "minimum recommended rule" of 10% but a tank that large should have plenty of room under the stand. Lots of people with a tank this size or so use a 55g for a sump that they install baffles in and portion off a fuge.