Measure the outer edge of the tyre bead to bead, take some string, mock up size/shape inflated by holding ends of string rim width apart and making a circle?
The tolerances are tight on this bike, my schwalbe g one tyres are allegedly 40mm but come up about 41.5 and just have clearance on the seatstays. Different tyres seem to assume different shapes when inflated up.
Measure the outer edge of the tyre bead to bead, take some string, mock up size/shape inflated by holding ends of string rim width apart and making a circle?