How much land does a land card represent?
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.