What is the longest in Canon or legends?
Assuming the lightsabers in Star Wars: The Lightsaber Collection are shown accurately to scale, the longest lightsaber from emitter to tip is the curved handle used by Asajj Ventress during the Clone Wars.
The Darksaber is marginally longer, but it's arguably not a lightsaber.
Keiran Halcyon has a very large (3 meter) lightsaber in I, Jedi.
I rotated my right wrist, twisting the throttle control up, and whipped the lightsaber around in a slash aimed to slice the deadHutt switch in two. With the twist I turned the lightsaber’s emerald out of the way and I brought the diamond into line with the Durindfire beam. This extended the blade from 133 centimeters to 300, narrowing it, but bringing the Hutt’s hand easily into striking range. Quick flick of the wrist, cleave the control in two, and the day would be saved. That would be the easy way.