The invention provides a device for measuring an inside diameter of a body lumen, such as an air passageway. The device includes a flexible catheter (110, 310) having an inflation lumen (112, 312), a fluid dispenser (102, 200) in fluid communication with the inflation lumen and operable to communicate a measurable fluid volume change with the inflation lumen (112, 312), and an expandable member (120) in fluid communication with the inflation lumen and having a known relationship between volume and a changeable transverse dimension, the transverse dimension being changeable in response to fluid volume changes of the fluid dispenser and arranged for placement adjacent to opposing portions of an interior wall of the air passageway (80). The expandable member (120) may be dimensioned for transoral placement into the air passageway (80, and may comprise a balloon that includes a complaint material or a non-compliant material.