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 Sram, the bolts must be 5 mm - 7 mm longer:

Technical Information:

Material: steel

Bolt Head: T25

Length: 15 / 27 / 32 / 37 / 42 mm

Compatibility: flat mount adapters