Thematically, how much land is a single land card supposed to represent? (It's for a story where the Magic: The Gathering card game is going to become kind of-sort of real through ignorance and stupid amounts of raw, untapped magical talent. ...yes, it is Harry Potter.)
Thematically a land card (or permanent) doesn't represent any physical land, but rather represents the planeswalker's bond with the lands that they visit as they move across the multiverse, they then draw mana through that bond in order to cast spells (although most of the modern Magic stories ignore this).
Every type of land in the multiverse has a specific kind of mana tied to it, and as such forming a bond with that land will yield that specific type of mana when you tap into it (hence the term tapping a land).
Source: the Learn to Play Magic videos.
Many of the more special lands depict a city, or only a single structure. For example Seat of the Synod, or Strip mine. Others show a very specific patch of nature, that's probably a bit larger than city-size.
Perhaps you could assume, based on this, that the other more common lands are about county-sized?