48"L x 18"W x 21"T
It's a good size for larger cichlids as it has a bigger footprint with a wide bottom. The 18" width gives a 12"-14" fish room to turn whereas the 12" width that is common on 60 & 55 gallon tanks is too narrow.
A 120 is a good size too as it is 48"L x 24"W x 24"T. Lot's of room to turn.
Of course the best size is 144"L x 48"W x 36"T but then again, we don't all have that much room in our house for a dinning room table sized fish tank!! :sad