The biggest eGo battery I have is a 650, but its too long to fit in a shirt pocket, or for stealth
vaping. That's for chain vaping in front of the TV in the evening ...
At this moment I have a mini eGo with a 420 mAh automatic battery (no button) and a pen cap cartomizer, clips in my shirt pocket and looks like a fountain pen ...
Hiding deep in my pocket is an Ovale Elips, only 4 1/2" long, with a 350 mAh battery ...
My 40 minute commute to work and back, heavy vaping at lunch time, and a couple or three "vape breaks", and my batteries are still going strong when I get home. I do keep a charged spare in the car, in case of overtime ...
The small batteries are fine if you're not hitting it constantly, and they are much easier to hide ...