It is narrower and shorter than a 510 using a cartomizer. The 306 is longer than a 306a. The 306a only has a 90mAh battery.
I originally tried to make things less complicated by having only Provarii and a hands free device. I move to wanting three diameters now. Wide would be the Provari, 14mm would be the medium size, then the tiny would be the cigalike. A 306a would be a kind of like a Blu without being reverse polarity. I am going to see if any of them were every made unregulated. 306a is classified as a super mini.
90mAh? Won't you be switching it out too often for it to truly be hands-free?