Short scythe (60 - 70 cm) is great for mowing in cramped spaces. Short scythe does not require much strength, which suits physically weaker mower. Poorly maintained land with hard plants is easier to mow with the short scythe.
Medium sized scythe (65 - 75 cm) is universal: suitable for gardens, orchards or meadows. Again, you have to consider mower's abilities.
Long scythe (75 - 90 cm) is great for cutting large and relatively flat land. Mowing is faster, but requires more strength.
The length is usually measured along the curved edge.