There have been a great many discussions about the USA being much larger or small in all sorts of more or less plausible ways. Oddly I have found hardly any talk about a seemingly obvious change: extending OTL's water border along the great lakes and St. Lawrence river all the way to the Atlantic.
For now I'm not so interested in precise PODs that could give this result. Let's just assume that the US was somewhat luckier during their independance war and managed to get a peace exactly like OTL except that the northern border is along the middle of the southernmost stream of the St. Lawrence for it's entire length, then hugs the south coast of the Gulf of St. Lawrence so the British keep most of the islands (incl. Montreal isles, Ile d'Anticosti, PEI, and Cape Breton Island).
This altered peace represents a bit over 100000 square kms additional territory, and a few 10s of thousands more people. The land is not especially great. There is some tin, iron, and coal. Forestry, fishing, and hunting will be the primary activity. The farmlands can support the area's own population but are not nearly as good as american farms further south. The population has a higher proportion of francophones and loyalists than the rest of the USA. Nowadays this area holds a population around 3 or 4 million, and some other minerals have been found.
What are the effects of this change?
How will this additional territory be administered? Perhaps New Brunswick is added as a state and includes large parts of OTL Nova Scotia and Quebec, and the rest of the territory is divided between New York, Vermont, New Hampshire, and Maine? Or becomes it's own state? Perhaps is temporarily a territory?
Does an increased size right from the start make the US more or less likely to survive?
These additional territories will presumably be considered part the New England region of the USA, and will have similar stances on slavery. How will a larger north affect the slavery debate?
The additional loss to the British empire is fairly small, but the loss to 'Canada' is enormous. How will this affect Canada?