The answer depends almost entirely on the type of food you take. If you stick with dehydrated or freeze-dried, use cous-cous for your pasta (or take a hammer to other, more bulky types of pasta to reduce its volume), eliminate bulky packaging and use calorie-dense foods as much as possible, you can get the size pretty small. I did a test two years ago on my Bearikade Weekender (650 cubic inches) and managed to cram in 6 days' food for both me and my dog. (As a rough estimate, about 10 liters.) That was leaving out the first day's food, and the 6th day didn't include dinner, which means that it was really 5 days' food. It was a really tight squeeze, though! My dog's food is 3 cups of kibble (relatively small pieces) per day. I did have to use a hammer on that and repackage some of it into smaller plastic bags so I could squeeze it into smaller nooks and crannies.
If you're estimating for pack size or bear canister size, just prepare one typical day's food for the trail and make up a bunch of other packages of the same size (crumpled paper in plastic bag) for the appropriate number of days.