The OM is the same size as a a Martin OOO.


                                
OOO (also OM)
Total Length        39 13/16"                39 5/8"                
Body Length        19 3/8"                20 9/16"                
Body Width        15"                15"                
Body Depth        4 1/8"                4 1/8"                


The Jumbo is 2 sizes bigger;

J
Total Length        40 5/8"        41 7/8"                        
Body Length        20 1/8"        20 1/8"                        
Body Width        16"        16"                        
Body Depth        4 7/8"        4 7/8"                        

For all the specs see here;

Martin Technical Specifications


Mark Ross