Hold down control, and play with your scroll wheel.
Or if you don't have an external mouse with a scroll wheel:
If you are using IE go to the view menu and then to text size.
If you are using Opera go to the view menu and then zoom.
If you are using Firefox, get Opera.
Edit: I beat you spike
Edit2: Spike is probably right and you weren't talking about in browser. So follow his advice, or see if somehow its running at the wrong resolution. Desktop right click -> properties -> settings.