Adjective used to describe a location or an object which dimensions perfectly satisfy its human inhabitants/occupants/users.

The interesting thing about it is the origin of its use : human beings have always tended to build things bigger or smaller than what they really need. Bigger things for Gods, Kings, and anyone who'd just come up with a big mouth or big muscles or a big bank account. Smaller things for Slaves, Servants, and anyone who'd just obey or earn its living from Mr Big mouth/muscles/purse.

Consequently, human-sized is an expression created to refrain from thinking too big or too small when building something. A kind of reminder, or at least a reference.