If you really want to go small but want to have a little more CO2 than a 12 gram then think about getting a 4 oz tank. Those things are extremely small and light. I'm not sure how many shots you can get out of one but I'd say that there is plenty there for more than 45 shots.
You can find 4 oz tanks at this website http://www.countypaintball.com
PS This is my favorite website for their excellent customer service.