Here's dimensions for an OE Replacement Muffler from Napa:
Body Length : 26"
Finish : Aluminized
Inlet Configuration : Offset
Muffler Body Height (in) : 8 1/4"
Muffler Body Width (in) : 11 1/2"
Muffler Inlet Connection Type : Pipe Connection
Muffler Inlet Count : 1
Muffler Inlet Diameter 1 (in) : 4"
Muffler Inlet Diameter Designation : I.D.
Muffler Outlet Connection Type : Pipe Connection
Muffler Outlet Count : 1
Muffler Outlet Diameter (in) : 4"
Muffler Outlet Diameter Designation : I.D.
Muffler Shape : Oval
Outlet Configuration : Offset
Overall Length : 37 1/2"
Reversible : No
Type : Combination