Martin guitars have a two part designation the body size a hyphen then the model located up inside the guitar on the block where the neck and body meet.
The model designations are 15, 17, 18, 21, 28, 35 and 45.
Models 15, 17, 18 post 1917 are Mahogany back and sides. The 15 and 17 has a mahogany top. The 18 style’s top is made of spruce.
Models 21, 28, 35 and 45 are rosewood back and sides with a spruce top.
The size designation at widest point (lower bout) measurement is as follows:
Size Width
2 12 inches
1 12 3/4 inches
0 13.5 inches
00 14 inches
000 15 inches
D 15 5/8 inches
Here is a link to my Martin Photo Gallery showing different Models and sizes.