In Kingdom Hearts, each of the Disney cartoon worlds is set on a distinct planet. Obviously the planets, as depicted here, are just icons displaying the design style and basic concept of the setting, not literal representations of the physical layout of the world. But the actual gameplay of Kingdom Hearts missions doesn't necessarily suggest worlds much bigger than these.
So, given that I want my games to be a bit more sandboxy, and less linear, how big should these worlds be in my game?
Well to start, I'll take note that, while animated films never have to define the size of their worlds, many videogames do. The playable area of Skyrim covers about six square miles. Eyeballing Skyrim compared to the other provinces of Morrowind, it's reasonable to assume a total land area of 48 square miles for the whole continent. Similarly someone tried to figure the area of the continent of Kalimdor (in World of Warcraft) and put it at approximately 41 square miles. Since the other continents appear roughly the same size, I think it's reasonable to assume a total of maybe 150-200 square miles of land area on the World of Warcraft itself.
There's no reason to assume every world has to be the same size, but I think these provide good upper and lower bounds.
Obviously, these worlds will be pretty tiny compared to the real world. But the point isn't to build a world that replicates real demographics and food-supply chains and so on, but a world that feels like in can support the cities and populations and cultural variety shown within. Cyrodiil and Azeroth may be undersized from a rigorous statistical viewpoint, but they don't feel cramped when you're in them. Similarly, any animated story would presumably be set in an equivalently sized world without sacrificing any of the story elements.
Not that I expect to need to map out every world in its entirety, but if the player characters decide to mount a large scale military campaign or something like that on a given world, I'll use these numbers as guidelines to develop the needed map or set a scale for a map that already exists.