I once drew a border estimating the largest possible Canada (with a more-or-less War of 1812-era POD), operating under the assumption that everything went absolutely right for Britain/Canada in border disputes (in a major departure from OTL). This also assumes an abortive Federal Republic of New England which rejoins British North America (nominally or otherwise), along with the Iroquois and then Tecumseh coming onside. Alaska was either purchased or seized from Russia (probably as a result of an alt-Crimea). You can probably assume that at least a few islands in the British Caribbean joined in as well. (Greenland, I'm not so sure about.)
I'm a terrible cartographer, so I used the watersheds map to help me with borders. The absence of the US-Mexico border is a deliberate omission; let them argue that one out over themselves. Anyway, as far as I'm concerned, Canada can't be much larger than this, otherwise it really isn't Canada so much as a Greater British North America.
Just a rough sketch I made once, and quite possibly wildly inaccurate, but I thought I might as well share it
I'm a terrible cartographer, so I used the watersheds map to help me with borders. The absence of the US-Mexico border is a deliberate omission; let them argue that one out over themselves. Anyway, as far as I'm concerned, Canada can't be much larger than this, otherwise it really isn't Canada so much as a Greater British North America.
Just a rough sketch I made once, and quite possibly wildly inaccurate, but I thought I might as well share it