As per size ref to regular cig... it's generally the "bigger the better" for ecig batteries. Atomizer/cart performances do all vary, but the battery itself, you really can't fool yourself with.
Thus, once you drop the requirement that the size has to be exactly a cig size... that opens up a lot of options for you. However, I'd say a good old dependable 901 or Janty Dura C/Joye 510 might suit you better - as it does have traditional cig look, but somewhat longer and fatter. If you drop LED, then it will open up whole new chapter too - as it can introduce to you to the world of manual switches. (Although there are some with manual and LED, if I'm not mistaken).
My smallest one was actually RN4081 - mostly known as Npro or SE Gold. It was small enough that was comparable to actual cigarette, and asides small tiny cartridge capacity, it could produce nice amount of vapor and good throat hits - but the battery was the problem for me. I ended up using only the USB Passthroughs for that model, until I switched to my Screwdriver.
However even with those smaller sizes.. I wasn't really holding it between my 2 fingers. It's substantially heavier, and it just felt strange. So I ended up using 2-3 finger ones like one smoking a joint or something. Longer ones like 901, was no different, and even longer ones like 801, I was actually holding it with my palm, just like what I'm doing with my Screwdriver.
But if you do think it's really important... see getting 2 units. 1 super mini like RN4081, and another little bigger like 901 or 510. It's good to have at least 2 working units, as one might go down and you have to resort to the other until the spare parts do arrive.
I had 3 - which was a bit of overkill for me. I had RN4081, 901, and 801. RN4081 and 801, I also had USB pass throughs connected with Kensington Power Pack. I sold them out and bought a single Screwdriver... and I'm getting a USB Pass Through from puresmoker for 901 when it's available for my back up.