it's in the name - 18xxx - indicates that it is 18mm in diameter. The last 3 numbers are the length - e.g. 650 mm and 350 mm. a 14500 battery therefore would be 14 mm in diameter and 500 mm in length.
How about the smallest 18650 tube mod. The Joyetech 18650 EGO-T? It runs in both EGO mode at 3.3v and unregulated mode right off the battery. The button is slightly inconvenient on the flat button model but it also features the 5 click on/off mode. I had 4 18650 2200mah panasonic batteries so I picked one up for 19.00. It rocks.
I have a LT and it works ok but kind of redundant if you ask me. And I don't particularly like the fact that it self regulates. The provari I have but it is nearly obscene to take anywhere without looking like a fiend.
Someone else mentioned the KGO. I can vouch for this kit being a good one. I don't care for the vari-volt version but the standard KGO lasts me all day at around 10 hrs using 1.7 ohm cartos or about 3-4 mils of juice. As far as working well and the basic requirements of most vaping habits, I'd put it up against just about anything. It's also conservatively sized comparatively and also has the 5 click on/off feature. It just runs direct off of it's 1100 mah battery, with very little voltage drop, and hits well almost all the way down to it's recharge indicator.
i own both and would recommend the new 1300mah spinner instead of a twist. its slightly thicker, shorter, and has alot more vape time than a regular twist.
the vape from a lavatube seems to be better to me at the same voltage settings, not sure if its placebo but i really like it. but the lavatube has all kinds of problems with the 510 connector, the button misfires, and if you get a chrome lt, its heavy. battery life with my v2 chrome young june sucks