If you are a hot hard dragger of full strength cigs or if you need long battery charge it may be best to skip the slim ecigs.
100mah is about 1 hour of moderate vaping
The two most popular slim ones are Joye brand 510 manuals (180mah) and Joye brand 510 mega xl manuals (280mah and about 128mm long with a cartomizer). Boge 3 ohm cartomizers work very nicely on these for your favorite thin juices.
For more hit and the option of prefilled cartomizers, look at Vapor4Life.com and SmokelessImage Volt (or Bloogplanet Maxxfusion, or Halocigs G6) - two styles of KR808D-1 and they can all use each other's cartomizers. If you plan to get several days to a week's use out of each cartomizer by adding eliquid, then I recommend manual batteries which are safer from being damaged or killed by juice leaks. Manuals also make it easier to get a good hit. The short sizes are 180 to 220mah, and the standard/78mm sizes is 280 to 320mah (the Volt style claims longer charge and may be slightly less voltage of 3.6v vs 3.7v). The standard size makes 120mm ecig with a cartomizers. For testing juices you can use a standard resistance DSE901 atomizer and drip tip on a manual battery. (only use cartomizers for juices you know you like. cartomizers have limited lives but are the most popular way to vape)
Both of the above models are a bit thicker than a cig, around 9.2mm, as are all decent slim ecigs.
For some popular fat battery models, go to Liberty-Flights.com, press the 20% off then look at the Ego models and Riva 510. These are 650mah and up and are big enough to optionally use the hotter low resistance atomizers and cartomizers. These can use most of the 510 and 306 threaded atomizers, Ego tank atomizers and tank, most of the stuffing and reservoir 510 threaded cartomizers.
All use eliquid.