Use the following as a good guide to measuring for a harness

Use the photograph as a reference for points to measure from, as this is a very well fitted harness

A:  Corner of mouth to corner of mouth behind ears
B:  Noseband
C:  Centre of eye to centre of eye (behind ears) Bridle crown fit
D:  Girth
E:  Back pad to top of tail
F:  Flank to flank (refer to location of breeching ring on photo) behind butt 
G:  Flank to flank - over the hip
H:  Back pad around front of chest to back pad