hobbyking and hobbypartz will both list the dimensions of the batteries. You'll find that lipos of similar capacity to your 6C nihm packs are generally quite a bit smaller, and some of the soft pack lipos are odd dimensions. If you get a hardcase lipo, they're generally the same length and width as a standard nihm/nicd pack, but may be taller. Just check the dimensions listed for whatever you're considering before you order. I bought two 4000mah 2s packs without looking closely (my first lipos) and they didn't fit any of my cars. Of course, the only logical thing to do was to buy more cars that they would fit in.. right?
I have this charger http://www.hobbypartz.com/skychb6chdup.html with no complaints, and have bought several batteries from them as well. Despite the warnings about exploding packs, you'll find that if you just charge correctly (correct voltage and amperage), lipos are easier than the old technology, plus they're soooo much better performance wise. good luck!