Here's a map of the rough provincial boundaries I'm planning and their modern OTL populations. Which ones do you think would likely be the most increased? (I'm guessing the Prairies for sure, and BC too).
Yes, Prairies for sure would go up, particualrly Peace, as Edmonton is well inside its territory and the oil sands being developed earlier (perhaps Britain decides to get to making them work in WWII?) would massively increase the population of that territory. (It's also not too bad in terms of land, either.) If you could get a Vancouver Island bridge built you could grow that number, too, and one could get Stikene-Haida bigger through making the cities of Prince Rupert and Kitimat somewhat bigger, and perhaps building a railroad to Alaska along the route of the Alaska Highway.
The National Capital Region, as others have pointed out, could be far bigger, and if Toronto is the bigger city than OTL I mentioned earlier than Ontario will likely be more than 6.8 million. Assiniboia seems somewhat on the small side for a bigger population Canada, too. To be honest, I don't see the point of Quebec not owning the northern part of its province, or at least splitting it between Outaouais and Cote Nord.
I'd be aiming for with those territories (Capital City):
Newfoundland and Labrador (St. John's): 875,000
Nova Scotia (Halifax): 1.85 million
Prince Edward Island (Charlottetown): 200,000
New Brunswick (Fredericton): 850,000
Gaspe (Murdochville): 600,000
Quebec (Quebec City): 3.2 million
Montreal (Montreal): 6.25 million
National Capital Region (Ottawa): 2.5 million
Ontario (Toronto): 8.75 million
Lower Canada (London): 7.1 million
Outaouais (Timmins): 1.2 million
Cote Nord (Sept-Iles): 300,000
Superior (Sault Ste. Marie): 900,000
Manitoba (Winnipeg): 2.5 million
Keewatin (Thompson): 200,000
Assiniboia (Regina): 750,000
Saskatchewan (Saskatoon): 1.4 million
Athabaska (Prince Alberta): 300,000
Alberta (Calgary): 4.5 million
Peace (Edmonton): 3.7 million
British Columbia (Kelowna): 6.5 million
Vancouver Island (Victoria): 1.7 million
Stikene-Haida (Prince Rupert): 325,000
Yukon (Whitehorse): 100,000
Northwest Territories (Yellowknife): 225,000
Total: 56.775 million