Trust me. batteries will be tiny and the rest of the e-cigarette will be back to its original form.
Battery technology isn't new, and there is always heavy development in that area. It's not done specifically for vaping, but we get the benefits from it. Most of the research is targeted towards cell phones, laptops, ipods, electric cars, and that sort of thing.
Since there is so much research, we can hope that there will be a breakthrough and we will have much smaller batteries with the same power as we are using now, or batteries the size we are using now which hold a lot more power. But that may not happen. Big companies have put a lot of effort into battery research over the past couple of decades. That means that most of the easy to find "improvements" are already done. And without a fairly big breakthrough, you aren't going to see huge improvements.
To answer the question, I'm using Vision Spinners, which are bigger than ego's, but smaller than lava tubes and provari's and such. I like the size factor, so if better batteries were available, I'd probably ant something the same size with more power.