I agree with the MCR suggestion - it's lovely and wee.
Satburn Mini is also a favorite of mine, but you need a 16mm atty for it. Mine is 18500 so I'm not sure how tall the smaller one is, but from comparing 18500 models it should be about the same as an MCR with a hybrid adapter. Mikro's Challenger (mk.III is coming soon) has a similar switch (designed by Satburn), but you'd need to find it in the classies or wait for the new version.
Microstick might also be worth a look.
The Nanos is very small and well machined... my only issue with it is that it will always fire if stood up unlocked. I know it's extremely easy to lock, but as all my other
mods have recessed buttons, I find I have to be very careful when using the Nanos. Otherwise it's great, and the price is quite reasonable. It's also important to note that the regular nanos is about 23mm (I think), so you want the Nanos Sleek if you want your attys flush.