Hey Jeat, welcome to ECF!
To be perfectly honest... if you want a reliable device with a good amount of power, you're going to have to make a *small* compromise on size.
I've tried a couple of the super-mini's (real cig sized), and even the good ones, the ones I liked, weren't quite enough to get me off cigs. And from all the experiences I've read here, the majority of people seem to find the same thing.
If you want a better chance of being able to kick it on the first try... I'd look at either a Kr808D-1, or a Joye 510.
The Kr808D-1 has the advantage of being a 2-piece design, so easier to figure out. However, when you factor in cartomizers, it is probably slightly more expensive. You can even this out a bit by re-filling and washing your cartomizers I prefer the flavor and throat hit of cartomizers - for me, it's the best I've tried, but there is a little less vapor.
The Joye 510 is a 3-piece, probably like what you've read about. There's more fiddling involved, but it's a very versatile device with great vapor, but usually less throat hit than the Kr808D-1. Because it's so popular, you can find parts just about anywhere, and I hear their atomizers are the best around.
They are both a bit longer and wider than a 100 cigarette. It's still cig-shaped, and trust me, you'll get used to the size very quickly. It's worth it for the performance they provide.
If you absolutely must have something that's exactly cig-sized, then the M401 models are the ones I've heard the best about. I've tried one, and it was nice. I liked it. But not as good as my Kr808D-1. And not as good as the 510 I've tried.
For the Kr808D-1, I had a great experience with ElectroNicStix. $35 for a kit, fast shipping, and Jason is really cool.
For the 510, ElectroNicStix also sells that, but I've heard nothing but wonderful things about CigNot as well.