Adjustable length lightsabers do exist in Legends.
According to the Legends lightsaber Wookieepedia article a lightsaber's blade was adjustable:
High levels of energy generated by a high-output Diatium power cell was unleashed through a series of focusing lenses and energizers that converted the energy into plasma. The plasma was projected through a set of lightsaber crystals that lent the blade its properties and allowed for the adjustment of blade length and power output.
However, blade length generally could not be adjusted quickly (e.g. in the middle of combat). A dual-phase lightsaber would be required to do so:
Unlike typical lightsabers, which often possessed a manual adjuster
for reducing the blade emission, the dual-phased blade could be
triggered in an instant, adding an element of surprise to catch an
opponent off guard. Additionally, dual-phase lightsabers also had a
An example of a dual-phase lightsaber is Exar Kun's lightsaber (which was double bladed). It was adjustable so that the blades were 0.5-1.5 meters long, and Exar Kun was known to adjust the blade lengths in combat:
Swift adjustments of either blade's length or intensity caused enemies to overextend themselves and allowed the Dark Lord of the Sith to counterattack with lethality.
Another person who used a dual-phase lightsaber is Corran Horn. He could double the length of the blade with a twist.
Also, some Jedi used a lightsaber shoto, which is a lightsaber designed to emit a shorter blade. Although these lightsabers were generally not adjustable, in the spirit of your question the short length could be advantageous by allowing different fighting styles. Examples of such Jedi include:
- Yoda (due to his small size)
- Ahsoka Tano (who used a regular lightsaber in her main hand and a lightsaber shoto in her off hand)
- Kavar (like Ashoka, he used a shoto in his off hand)
- Maris Brood (who used a pair of lightsaber shotos with tonfa hilts)