Take a regular sheet of paper. 8.5x11 Standard Copy paper.
Fold it in half via hotdog style, or long ways.. So its still 11inchs long. That is basically the size of the card. Like Really. I'm not joking.
IF that doesn't help, I stuffed one in a CoolerMaster Elite (mid-tower) with a MATX board. and I didn't have any problems... however, it was close to the processor and it was a top PSU mount so the heat blowing down from the PSU was landing straight on my card.
Also, I would warn in a smaller case you are going to have a harder time cooling everything since the card gets hot.