This really depends on the template. Since I wrote my comment in 2009 we got a new default template with which you can do what I wrote. There are various ways but you can put this into your userstyles to change the size of everything:
body {
font-size: 100%;
}
(For example, this changes the base font from the equivalent of 14px to 16px.)
But the Writr template is based on px and is also mixing different units, so is not flexible enough to support this. When you change the font size that way, it will change the size of some elements but not all, which will make it look broken.
If you're asking for other templates, it really depends. It could work with
body[/m] or [m]div.dokuwiki[/m] or not. And it could work in general or it might not. You might need an [m]!important, you might not.
Sorry for the bad news.
So, if you'd like to know how it works in your template or if it works at all, you will have to tell us which template you are using or give us a URL to look at.