The ATX12V Power Supply Design specifications defines the standard physical dimensions as:
150mm W x 86mm H x 140mm D
or
5.9"W x 3.4"H x 5.5"D
The Height and the Width are fixed.
The only dimension that is allowed to vary is the depth. Higher power PSUs will have a larger depth.
E.g.
Seasonic Platinum-400 Fanless (SS-400FL2 Active PFC F3)
Dimensions: 150mm W x 86mm H x 160mm D or 5.9"W x 3.4"H x 6.3"D
Seasonic Platinum-1000 (SS-1000XP Active PFC F3)
Dimensions: 150mm W x 86mm H x 190mm D or 5.9"W x 3.4"H x 7.5"D