For training I usually stick to 23c, but if I can find 25c tires on sale I'll slap those on just the same. For reference, I weigh in around 63kgs. The larger tread gives you some extra grip on slick roads, not to mention that when exploring Tokyo/Chiba back roads I often find myself on dirt sections that I would never want to ride on 18c-20c! Also, I use rim strips (an extra layer of protection against flats) that you cannot easily fit in w/ the tube when running sub 20c tires.
Before running extremely narrow (sub 20c) and/or high pressure tires (125psi+), make sure you have a set of rims that are spec'd for this kind of tire!!! Some of the cheaper wheel sets out there are not necessarily built to handle high performance tires. I once blew out a 19c Continental going through a tight turn in a criterium and found myself in a world of hurt on the pavement. The reason: the rim was not built to run kevlar beaded tires at 150psi!
I have yet to venture into the world of tubular tires, so I suppose tire size/rim selection will differ a bit versus clincher.
It's safe to use narrower tires on the same rims? What's the limit?
Before running extremely narrow (sub 20c) and/or high pressure tires (125psi+), make sure you have a set of rims that are spec'd for this kind of tire!!! Some of the cheaper wheel sets out there are not necessarily built to handle high performance tires. I once blew out a 19c Continental going through a tight turn in a criterium and found myself in a world of hurt on the pavement. The reason: the rim was not built to run kevlar beaded tires at 150psi!
I have yet to venture into the world of tubular tires, so I suppose tire size/rim selection will differ a bit versus clincher.