ecigarette-mods.com is the definitive guide to
mods.
This chart lists them by battery type and voltage. I'm not sure which is the most slender; perhaps other vets will chime in on this.
NOTE: Since a 6V device requires stacking
batteries, I strongly recommend getting a PV with plenty of venting (and/or other safety features) and only using protected
batteries.
BTW, are you
sure that you want
6V? 5V (or 3.7V with an LR atomizer) might meet your needs very well. That opens up many other options, including slimmer PVs and the variable voltage Buzz (which
does go up to 6V).