A = Back length. Measure the length from the withers to the ears.
B = Side length. Measure half way up the neck, the length from the mane to the bottom of the neck.

Order size
Top length (A) Side length (B)
Large - - - - - -
Medium - - - - - -
Small - - - - - -
XL - - - - - -
Order size Large
Top length (A) - Top length (A) - Top length (A) -
Side length (B) - Side length (B) - Side length (B) -
Order size Medium
Top length (A) - Top length (A) - Top length (A) -
Side length (B) - Side length (B) - Side length (B) -
Order size Small
Top length (A) - Top length (A) - Top length (A) -
Side length (B) - Side length (B) - Side length (B) -
Order size XL
Top length (A) - Top length (A) - Top length (A) -
Side length (B) - Side length (B) - Side length (B) -

All sizes in the size chart are measured with the utmost care. The sizes may differ slightly from the actual product. You cannot derive any rights from this size chart.