The Jedi don't use shields because there aren't any in universe that would work against a lightsaber
Deflector shield technology in the Star Wars universe is not capable of producing a man sized field that can be powered by a man portable power source that can stop a lightsaber. The closest thing they have is the smaller fields that can hold a door against the vacuum of space, but they are not portable, and Jacen and Jaina have disabled them before with a lightsaber (Young Jedi Knights series)
What about the Gungan shields
During Phantom Menace we see those shields failing under blaster fire, something we know lightsabers are directly stronger than. It wouldn't be worth carrying a shield around that can only withstand a few shots, especially since it have to come with a trade-off in maneuverability.
with those special cases resolved, lets move to the real answer -
You can't turn a light saber into a shield, the physics of that universe don't work that way
The important parts of the lightsaber for this answer are the primary crystal and the focusing crystal. Without both of these, when you turn on the lightsaber, you get either a explosion or a slagged lightsaber casing (Young Jedi Academy books, Jedi Academy books). (a smaller explosion than a thermal detonator, so still not militarily applicable). We also learn from Corran Horn, Jedi master that when you mess with the length of the blade it grows weak and unstable (I,Jedi and the yuzhong vong books) to the point that striking anything is enough to able to short circuit the beam.
Basically, the reason that there are so many lightsabers that are exactly the same blade length is because that is the length that they are the most effective at and most Jedi don't find the trade offs worth it
One other small nitpit, that makes sense to me but I don't have a source for, is what material exactly are you going to use as a splash to divert the energy outward in a shield shape? Lightsabers cut through anything. Some materials are resistant but none would hold up to repeated use. Who wants to take equipment into the field that might fail at any moment? Like I said, no source but without a way to do that, I don't see how a lightshield would even be possible.