L1 - 4.6 inches
L2 - 6.1 inches
E2L - 5.25 inches
For me, the L1 size is perfect. 6.1 for the L2 is a bit long, and the E1L is too short (in my opinion)
The L1 and E2L use the same emitter (someone correct me if I'm wrong). The E2L uses the KL1 head, which is essentially the same as the L1, but the electronics are in the body of the L1, where they are in the head of the E2L. The E2L has a little more output, but that is the only difference. They use the same optic.
The L1 is one of those lights that on paper isn't very impressive, but I find it more and more useful the longer I have it. With the F04 beam shaper (which is a must), you have a very versitile light that keeps running and running.
You can always buy one of Milky's ML1 heads too and then you have ~60 lumens output and great spill.
I have U2 as well, but I find myself using the L1 more often as the 1.1 lumen low setting works great at night and for closeup stuff and the high setting is bright enough to check that dark corner wherever.