I loved slim cigarettes. But you aren't going to find anything slimmer than a Marlboro. About 8.5mm diameter.
I use M401 and M402 manuals and find them pleasingly slim. M401 is the longer length and longer lasting battery and is 108mm long (around the length of a 110 cigarette). These models have very small cartridges you have to mess with a lot. You have to use an unbent paper clip to push the inner cart up out of the mouthpiece then top it off with
juice fairly frequently. When the cart starts not absorbing well, I remove the inner stuffing and pat it between paper towels, out some
juice in the cart (about 2/3 full) replace the stuffing carefully, get it positioned right, top off with a drop or 2. When the cart gets too finicky, I wash it and replace the stuffing with blue foam (the blue foam
mod). I got my custom kit here w/ gold atomizer, one black/gold design manual battery (blue led) and one white manual battery (orange led). Not quite the best price but I liked that I could customize the kit.
I believe his discount code is 10FOR2010 for 10% off.
http://www.mypvshop.com/store/electronic-cigarettes/m401/
He doesn't carry the M401 PCC (charging case) and is out of stock on the non-charging M401 cases but you can find them elsewhere.
The M401 is considered a lightweight around here. Many people prefer popular minis (longer and a bit fatter) and larger units like Ego and Riva which look more like fat magic markers than cigarettes. But at least the M401 is a respected unit. I've found the fit and finish to be A+ and I don't mind screwing with the tiny carts frequently. I would not recommend it to anyone who is unable to do that.
Besides Mistress Nomad's videos, I recommend you read the multipart illustrated guide in the Library sticky thread (top of this newbies section), especially part 4 which starts getting into specific models. You'll see the M401 mentioned in the guide. And BTW - most people recommend manual batteries. Automatic batteries can turn on accidentally from wind and vibration.