Best look numbers on heel outside:
Here are the outside shell dimension that you'll find molded into the area of the boot just above the heel pad (medial side or inner ankle).
SB Shells (Track 325, Track 225, LeMans, SB### Series)
M22 = 253mm
M23 = 261mm
M24 = 269mm
M25 = 277mm
M26 = 285mm
M27 = 295mm
M28 = 303mm
M29 = 313mm
M30 = 322mm
M31 = 332mm
AF Shells (Track 700, Indy, Suzuka, AF700/600)
M24 = 270mm
M25 = 279mm
M26 = 289mm
M27 = 299mm
M28 = 309mm
M29 = 319mm
M30 = 329mm