For rear flat mount calipers, you must measure the frame thickness (the diameter of the tube where the bolt passes through) to select the appropriate bolt. According to manufacturer specifications from Campagnolo, the standard pairings are:

19mm Bolt: For frame thickness of 10–14mm

24mm Bolt: For frame thickness of 15–19mm

29mm Bolt: For frame thickness of 20–24mm

34mm Bolt: For frame thickness of 25–29mm

39mm Bolt: For frame thickness of 30–34mm

44mm Bolt: For frame thickness of 35–39mm