Its between Medium and Extra Large - Duh
The Gallon system is American and a simple Euro scale is S- XXL. But I would equate the volume of the Mad Boy to be about 70 gal. Fish is 55, CriticalMass is 84, Mafia is 72, Gangster 62, Squashtail 53, Stinger 56, Thruster 58, Pintail 59 to compare