Page "Outboard motor" Paragraph 32

Outboard motor shaft lengths are standardized to fit 15-inch, 20-inch and 25-inch transoms.

If the shaft is too long it will extend farther into the water than necessary creating drag, which will impair performance and fuel economy.

If the shaft is too short, the motor will be prone to ventilation.

Even worse, if the water intake ports on the lower unit are not sufficiently submerged, engine overheating is likely, which can result in severe damages.