Recitation: I bought an eGo Twist 900mAh battery. The girth is about the size of a cigar, and the length varies based on the mAh rating of the battery ranging from about 3" to 5" or so on the larger eGo batteries. I've tucked mine in the back pocket while walking to work, didn't even notice it and would notice it even less if it was a shorter one like the 350mAh or 650mAh battery.
Informative statement: My 900mAh eGo battery with a clearomizer attached is about as long as my hand, from the wrist to the tip of my middle finger. I'm a male, but I don't have overly large hands. Only other things. It's very unobtrusive. There are many options when it comes to smaller models that perform well. If you're going to buy one of those type, it's probably better to go with the VV models (Twist/Spinner) so you have that option in the same small package. A 350/650mAh eGo Twist with a clearomizer should probably provide a good 3-4+ hours of medium vaping and not even have to change the clearomizer, so would make an excellent option for at work or a night out without being very large at all. Would be even smaller with a cartomizer, but a cartomizer might not hold enough juice to carry you through a night out. Of course, you can always carry different cartomizers with different flavors as they are small and convenient.
Elaboration: Clearomizers come in two styles, top coil and bottom coil. The top coil have wicks which hang down from the atomizer at the top. They tend to work better with thinner juices as with heavier juices they have wicking issues and you have to tilt/splash them more perhaps. A bottom coil provides a cooler vape, but has less of these problems. I haven't had any issues with my top coil or bottom coil clearos so far, but I'm also using 100% PG juices which are thinner.