Guys, we need to clarify this.
There is no regulation that says that everybody on the JMT must carry a bear canister. The regs to say that the food that is carried must be contained within a bear canister in certain areas. Plus, the food carried on Day One is generally consumed by the end of an evening meal on Day One, so it did not need to be within the bear canister during the middle of the day.
Now, let's suppose that there are three men in the group. When the ranger stops you along the trail, he will ask to see your wilderness permit. When he has that in his hands, he will ask you where you are coming from and where you are going to, and that better agree with the permit. Then he will ask if you are using bear canisters, and the answer better be positive. Then he goes back to where you were coming from, and where you are going to... in how many days?
So, let's say that between the start and a resupply point you tell him it is five days. Five days times three men is fifteen man-days of food. At this time, he asks to see your bear canisters. If you have one humongous bear canister that can hold fifteen man-days of food, and if he is convinced, then you are good. However, if you have some lesser solution or something that just does not compute, then you are in trouble.
Now, if you are a long-range thru-hiker, and if you have a permit showing this (covering more than 500 miles), I think you get an exemption, and thru-hikers are shown to the bear boxes, like at Rae Lakes. However, there may or may not be room in them, so that is not a foolproof solution.
For a ballpark figure, start with 100 cubic inches of bear canister volume for each man-day of food. For me, I require a little more, like about 120 cubic inches. If you have small appetites, maybe you can cram your food into less volume.
However, it is not me that you must convince. It is the trail ranger.
It is just a whole lot more practical solution for each person to carry his own bear canister and food, but there are alternatives.