John E

That's really up to you. If you currently have a 13 or 14T high, you can go as small as 11T for a dramatic increase in your high gear ratio. (This is much higher than anything I would ever care to have, but plenty of new road bikes come stock with a 53/11 top gear.) If your current low gear cog is something like 22 or 24T, your rear derailleur may be able to take a larger low, such as 26 to 28T, which will give you a noticeably easier climbing gear. Changing from a 13-24 6-speed to an 11-28 9-speed will widen both ends of your range without compromising your progression.