Another possible solution is that you've changed the screen resolution. If your desktop icons are still the same size, then it's the Windows font size that's changed. If *everything* (icons and text) are bigger, and the display looks a bit "blocky" too (individual pixels are easier to see), then it's the screen resolution.
Sadly, the screen resolution often gets changed for you, at least on my old Win98 machine. If I don't have my monitor switched on when Windows starts, Windows doesn't know what resolution the monitor would be able to manage, so it goes to some "fail-safe" resolution that any monitor will manage, and you have to change it back manually. This could be the problem you had. If you find this happens to you, just remember to always turn the monitor on first. However, you still need to get the resolution back again...
Either way, right-click on an empty area on the desktop (ie. not somewhere where there's a window or an icon) and select "Properties" from the menu (or go to "Control Panel" and select "Display"). Go to the "Settings" tab.
If it looks like the problem is the screen resolution, there's a slider marked "screen resolution" that you can tweak. Usually you'll want this all the way to the right.
If the resolution has been changed automatically by Windows, the screen refresh will probably also have been changed to some low-spec value. This will show as a visible "flickering" on the screen, similar to TVs, if you have a traditional CRT monitor. To fix this, hit the "Advanced" button. On this new window, one of the tabs (either "Adaptor" or "Monitor") will have an option for "Screen refresh rate". You want to set this to the highest value possible. If you have a flat-screen monitor then the screen refresh is not a problem, so don't worry about it.
If the problem was the font size, there are two places that it could have happened, at least on XP. On XP only, the display properties window has a tab "Appearance", which has an option "Font size". If this is set to anything other than "normal", change it back. For the other place (the only option on other versions of Windows), go to the "Settings" tab, select "Advanced", and on the "General" tab of that window you'll see an option for "DPI setting". Again, you'll probably want "normal" rather than "large". Note that this will restart your PC if you change this, so make sure your work is saved first.
That pretty much covers it. The only other remote possibility is the Windows theme, but that's so unlikely that it's not worth worrying about until you're tried the other ones.