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35c easy, 38c measured width fits as well but you'll need to be quite far back at the dropout to not rub the seat tube, also limits you if you run a different sized cog on the other side. Worked absolutely fine for me with 47x19 though. If you're mudguard curious, it is possible* to fit 45mm mudguards, although with that I'd recommend no more than 34c measured, I have 32c with mudguards and have bags of space.
*but more than a little bit tricky.
Hmmm, what's the tire clearance on these?