PURCHASE A LARGER SIZE
I bought a medium size for a medium sized breed, that in the dimensions, should have fitted. Upon arrival, the harness was slightly too tight! Purchased a one-up size (large) which fits perfectly.
A great all-round harness, however, I find the harness worn backwards seems to fit more comfortably, (front lapels on the underside) allowing for a better fit/more comfort.
I will continue to use this harness, however may not be suitable for show dogs, due to it's hair flattening effect!