The
Mini-T from Super T will be quite slim, but it's not available for a while, as the Lady pointed out.
Your best bets at this point for a slender, compact shape from a well-reviewed company are the
Protege from Puresmoker, the
eGo from Janty, or the
BB from Altsmoke. (The BB can also do 6 volts. I was scared off by reports of an easily-scratched finish on it...then again, I always have pliers and sharp scratchy tools flying around my workspace.

) I read up on the Protege and the BB when I was considering sticking with 3.7v, and looking at the eGo now that I'd like a small unit for going out...all
three have their fans.
Choose based on the other features of those three - they vary in color, button, etc. - and from there you're set to go.
(Note: I chose the Xhaler myself for a first
device even though I wanted something portable and small - simply because I didn't want to lock myself into a 3.7v
device alone. Turns out 5/6v is amazing, and the Xhaler's at least portable enough for the car. And the button placement at the end is far less of a problem than I thought it would be. But, only my experience.)