small room; closet; cubicle; booth; stall
Refers to any small enclosed space within a building, such as a tiny room, closet, booth, or stall. The exact meaning depends on context.
See also: 部屋
This apartment has only one small room.
I bought a ticket at the station booth.
小室 is a slightly literary/technical word for a small compartment or chamber, while 小部屋 is the everyday word for a small room.
Compound of 小 (こ, 'small') + 部屋 (へや, 'room'). The reading こべや is a straightforward combination.