It is all well and good to propose scenarios with a larger British North America. I can easily imagine a scenario where the British seize what we know as the Midwest, the Pacific Northwest, the Louisiana Purchase, Alaska, and parts of New England. Heck, if you can get that far, I can buy that they would go for California, New Mexico, and Texas as well.
The problem is... well, once independence is achieved, is this country recognizable as Canada, or is the land that comprises Canada in our timeline ironically a largely peripheral area of a continent-spanning empire with its population and political, industrial, and economic power based elsewhere? In that case, you may as well have the United States annex Canada and say that this counts as greater Canada.
In my opinion, the largest that Canada can get while still remaining recognizably Canadian by our timeline’s standards would amount to an addition of Alaska, Maine, the Pacific Northwest, and the northern portion of the Midwest - roughly the Dakotas, Minnesota, Wisconsin, and part of Michigan.
EDIT: I see that
@Jackson Lennock already covered much of what I wrote.