No they actually do make HUGE (compared to 5mm diameter LEDs) 3/4" LEDs
And yes I believe someone has tried it in the past for use in a padawan type saber.
It won't illuminate quite as bright as the typical 3W LEDs we tend to use in custom saber building, but for a padawan saber (i.e. with a relatively short blade), it should work just fine, particularly at night, and particularly if you use a green color (which our human eyes tend to perceive as "brighter")
For a Kylo Ren (presumably red colored), I'm not quite sure it'll illuminate the way you would like it to. Even the 3W LEDs we use can be a touch washed out during the daytime.