A combustor assembly for a gas turbine engine comprises a combustor casing, and an integer quantity N of fuel swirl nozzles. The combustor casing defines a total internal volume of the combustor casing V (cm3) together with a Fuel Swirl Nozzle Density ratio (m−3) that is defined as: D F ⁢ S ⁢ N = N V and the DFSN parameter is a value in the range of 200 to 1,500.