I don't think they say! I've looked and looked at mine and cant find it anywhere, I even took it to a local saddler who couldnt find a width but was able to tell me it was 99% likely a medium fit so maybe try that if you have a saddler locally?
Is it an adjustable saddle where you buy different size gullets to fit your horse? If so, I wouldn't think there would be any mention of width size on the actual saddle.
mine is a fixed tree and it is printed on the girth strap cover. I do know others who have GFS and it's not printed anywhere (maybe as SB mentions they are adjustable ones)
Any wooden tree (and some synthetics ilke Kieffer) can be adjusted so even if the width IS somewhere on the saddle (sometimes on the stirrup bar itself) you can't rely on it.