The original IkenVape Chameleon was a really nice small light 16340 MOD. He doesn't sell that model anymore, but an updated replacement for that model is due real soon now (i have my eye out for this one). The SuperT Presice16 is also small, and more stylish, but not as light. The MadVapes Mini VapeStack is another, it's short and squat rather than tall and slender. Those are all of the small tube MODs that I've used. My #1 is the P16 because of how finely made it is and how great the switch is, although I do wish it were a little lighter (and this is why i have my eye on the new chameleon).
The GLV2 super mini looks pretty good, and the Steampunk Sparkplug SP2, and the XS-haler.
There's also the PreciousM (14500), and the Mako line of mods (although the smallest uses too small of a batt to be useful for me), and the Indulgence (if you can stomach the branding).
Lot's of listings here...
Battery Tube e-Cig Mods | e-Cigarette Mods Database
... omg... so many options!
If you go up to 5v, you'll have to go up a lot in size to accommodate 2 batts instead 1.