technically, a hyphen is shorter than an en-dash, but even less people use en-dashes correctly than use em-dashes correctly. Ideally a minus sign is different again, though it isn't always.
An em-dash is so called because it is generally about the width of a letter M, which is generally the widest letter, and the en-dash is about the width of a letter N, so an em-dash is usually about twice the width of an en-dash.
As usual, wikipedia contains far more than most of us would wish to know http://en.wikipedia.org/wiki/Dash