For me, a 3-night/4-day trip is at the dividing line between the small and medium canister. What a lot of people here call "large" is really the medium canister. The "large" has the same amount of fuel as two medium canisters. I use a small for 1-3 nights, medium for a 4-7 nights, and large for 10-12 days. For that odd 8-9 day trip I take one small and one medium. I also really cook, not just boil water and like several hot drinks when nights are cold.

I would take the medium at first. After you have backpacked enough to know your needs, you may be able to size down to the small. I seldom use the small canisters because they cost nearly as much as a medium and it is really nice to have extra fuel for more hot drinks or even to warm water to wash my face! The Jet-boil system uses a heat exchanger and if you have this type of system, a small canister should do fine. However, the added weight of the heat exchanger pot just about evens it out in total weight. Experienced backpackers are able to squeeze more cooking out of the small canister, but as a beginner, you may not be as efficient.