I just got my 650mAh Twist today and its the same size as my 1000mAh kGo. I would have gotten the 1000mAh one, but I don't like how long it is and I'm a fairly light vaper so it doesn't matter as much.
So far, I think this thing is great! I'm using it with a Stardust and I've been having fun experimenting with the voltages. I got it because I was curious about VV but didn't want to spend a ton of money or have a huge PV. So for me, this was perfect as far as the size and price goes. One thing I don't like is the plastic indented button, but that's not a big deal.