There are three popular sizes which are standard stock items and the dimensions are as follows:

Intermediate 32"L x 22.5"W x 24.5"H

Large 36"L x 25"W x 27"H

Ex-Large 40"L x 27"W x 30"H

Length and Width are measured to the outer lip of the central flange and the height is reduced by 2" when the false floor is fitted.