2cm can make a huge difference for your knees. If you need a 58, then don't go smaller. You can only raise you saddle so much before you hit the minimum insertion point. It happened to my husband and don't think that wasn't an expensive deal to fix. Of course, he wasn't terribly disappointed to have to buy a new bike since he used the opportunity to upgrade to a higher model.

You might also want to take some measurements and use a website fitting program to at least get an idea of what size and various measurements might fit you best. There is one on coloradocyclist.com. There are others, but my brain can only recall that one at this moment.

Happy hunting!