four-and-a-half tatami mats; four-and-a-half-mat room
Refers to the size of a room measured in tatami mats, or the room itself. A classic small Japanese room size, often associated with student lodgings or humble living.
He lives in a four-and-a-half-mat apartment.
A four-and-a-half-mat room is small, but it feels cozy.
small room for assignations; love nest
Historically, a small room used for secret meetings or romantic encounters. This usage is now somewhat dated and literary, often evoking a sense of nostalgia or seediness.
In that novel, a four-and-a-half-mat room is depicted as a place for secret meetings.
During my student days, I lived in a four-and-a-half-mat apartment.