42mm is the stated range. I have 40mm (tubeless) on there now and there is plenty more room.

After giving this more thought I'm probably going to skip a handlebar bag for my credit card touring purposes. With my 2.5L frame bag and a tool carrier under my downtube, if I add a S/M sized saddle bag...and maybe a 1L bolt-on bag for my top tube, I believe I can carry everything I would need (for 2-3 days at most, washing bike clothes where I'll be staying and drying them overnight). These rides are during the summer and I only need a change of clothes, cheap/light sandals, toiletries, and a few other small accessories. What I like about these rides is I leave from home, take many stops along the way (plenty of services for food/water), arrive kind of late and don't really need a lot. Simple adventure!