I bought my old Colnago in Italy, complete with sewup tires which I rode on the cobbles. Then another bike a few years later, also with sewups and more cobbles.

It has been a while, but there were two basic types of cobbles. The little ones, maybe 4" square. They generally seemed to be well made, and never were a problem for me. There were also the big cobbles, maybe 1' x 2' granite stones set at a diagonal. Some would rattle when I rode over them (at breakneck speeds as I was younger then). The big problem is that they had gaps that were probably the size of my tires between them, but I never had problems. 6" to the right or left could have been death... but I kept the track right down the center.

The little cobbles never bothered me much, although I think the cracks tended to hold the glass worse than other types of pavement, so I ended up with several flats during the couple of days of riding in Rome. At that time, I had managed to get a big supply of pre-flatted sewups for cheap, which I repaired and rode until the next flat and repair.

If I was doing it again, I'd probably still ride my 23/25mm tires, but might consider bumping it up to 28mm. However, I would not choose big MTB tires. Flat protection would be a huge benefit for some areas.