LAST EDITED ON Jun-24-13 AT 03:24 PM (EDT)
 
Well, Lightsabers depend on certain crystalline optical components, the dimensions of which seem to dictate those of the blade. The likelihood of a crystal being unusably flawed probably increases exponentially with its size. You could have an unreasonably large Lightsaber, but it would take a lot of patience and willingness to suffer exploding sabers.

EDIT: and this is why one should read the entire thread before replying. Oops.