Small in diameter, or small in length? In diameter, with full VV-VW functionality at a very reasonable price would be a Innokin iTaste 3.0 and any number of clearo/glasso atomizers.
In a short mod, with modest diameter... well, not so many. I'm not even sure what's in the photo is an actual battery, on the bottom - if it is, it's very short, and would have limited mAh capacity - and it appears that it
might be some sort of tiny mech mod with no outward signs of regulation or control. I'll be curious for someone to identify it.
An 18350 mech (or limited control device like the Innokin Cool Fire) as mentioned by Nikkita, or a short, 350/400 mAh eGo as proposed by Dave... those would certainly be candidates for "short".
The problem with small will always be, limited mAh, limited to no regulation - or both. Pricing can be all over the range - from the $8 mini eGo, to the
Titanide Themis 10 or 14.