Islands

 When asked what the population of Medieval Canada was, I answered "Somewhere between four and five million". From what I can tell, "central" Canada (Ontario/Quebec) has about three million people, and western Canada about a million between them. What we don't know for sure is how many people live in what's know as Atlantic Canada, or the Maritimes. 

In fact, the Medieval America project overwhelmingly explores what we would call the Continental United States, and it's closest borders. Occasional maps will give us a glimpses of the most adjacent surroundings--we know that British Columbia is part of the Northwest culture, Cuba and the Bahamas are part of an overall Gulf culture (Secretarial States and whatnot), and that Ontario, New Brunswick and Nova Scotia are very much assimilated into the American Church (and the latter officially absorbed into the United States). We know a little less about Quebec, but we do know it remains as a political fiefdom and remains Roman Catholic (Likely leaning into it, resembling its Colonial days). 

White does not cover areas that are much further out, even if those areas are currently part of the U.S., some states, some not. This is probably to be expected, as the maps would have to be extensive, and covering a lot of "dead space" like ocean, or unpopulated wilderness. Alaska, already not filled with a lot of people, and most living in cities propped up by modern infrastructure, would most likely simply revert to the indigenous tribes that lived there for thousands of years, and shake off the past 200 years of developments like a thin coat of paint. Hawaii would probably be a little less isolated, as an important pit stop in the Pacific Basin. However, as the cities crumbled into nothingness, it's very likely the Native population, which has never fully accepted the coup orchestrated by the American mainland, would almost immediately divorce itself from the United States from a political or cultural standpoint. It would ultimately not be part of "Medieval America", except in any abstraction. Puerto Rico, which has been talked about for Statehood, would probably be more outside the American sphere than say,  Cuba, but it's pretty easy to imagine it's culturally very much like southern Florida.

And then there's New Foundland. One of the quirkier parts of Canada, with its own accents and culture.You could say it's not part of Medieval America. You could say it's not even a part of Medieval Canada. It did not even join Canada until the middle of the 20th century (After World War 2). It has its own time zone. While it's not too far off the coast geologically, the shortest boat ride is off the absolute most northern part of the coast. Most of the people, most of the culture, most of the activity is on the Eastern half of the Island. In Medieval America it's possible they only barely interact with Americans more than Europeans. (Most of the interaction would be the fairly limited pilgrimages and trade between the two continents.) It's probably unlikely they're able to be the source of Neo-Vikings, because the Northeast of America actually has the best centralized navy (It's quite possible they tried out Viking style raiding, and the U.S. nipped it in the bud. It's also possible the U.S. was the aggressor from the start, keeping New Foundland from growing its own barbarian troops.) The population is probably not very high--right now it's about half a million, but like Maine, it's probably not going to experience too bad of a crash, either. But however many people live there, when White talks about the population of U.S. and Canada having 61 million people, New Foundland is not part of those stats either way. It is its own separate thing.