The shaft length is measured from the inside of the transom mount down to the cavitation plate above the prop. The accepted short length for Johnson Evinrude
etc is 15" but Tohatsu outboards are slightly different from most others... the Tohatsu is 17". This was the main reason why I have a Tohatsu 8hp on my 25 foot sloop
... I get those extra 2" without having to have a long shaft outboard
Also, It is worth investigating the power rating as many outboards can be economically uprated. For instance, the Tohatsu uses slightly larger carburettors to go from 6hp to 8hp to 9.9hp... identical otherwise.