The most convenient SI unit would be the centimetre.
You should choose your unit so that the numbers in the measurement are between 0.1 and 1000.
Thus, the package might measure 14.2 cm x 12.5 cm.
Other acceptable measurements would be 142 mm × 125 mm and 0.142 m × 0.125 m.
You could even use 1.42 dm × 1.25 dm, but the decimetre is not a widely used unit in some parts of the world.
