49 cm would worry me too.
More seriously, the way Reversos fit really depends on the shape of your wrist, more than the size. If the top of your wrist is flat, you can trust your cardboard mock-up. If not, you really need to try one.
You may find guidance in the lug-to-lug size of your usual watches too, but I wouldn’t buy one without having spent a good 30 minutes in a JLC AD.