Agree, agree, agree and agree lol.
Almost every tube mech comes with an 18350 tube, either included or available seperately. Size is completely dependant upon each device.
As I already suggested, the Zenesis2 DX350 is a short little mod. For me, any shorter would be bad, itd be uncomfortable. The DX350 is right on the line of being too short, but just enough that its still comfortable to use. And of course, down the road you can add the carto tank or genesis atty and turn it into a hybrid. I have a Zenesis Altus that I sometime take the atty and put on the Dx350 tube.. The colors dont match because gun metal and brass, but if I used it like that all the time I could just get the right color top cap or body tube. I know I sound like a fanboy for Zenesis, but the quality of these device is beyond what you expect, and the versatility because of the common threading on all the possible parts makes it a great system to get into.
In the Hybrid department, there is the Noble 1. An aggressively priced genesis hybrid which uses an 18350 battery, it is quite small, and an overall nice mod to boot. Comes in Stainless and Black.