it's probably not the font size you need to decrease but the screen size you want to increase.
To do this right click anywhere on the desktop and select properties on the menu that comes up, on the properties dialoge that then comes up click on the far right tab that says settings. on this screen there is a slider marked screen resolution, move this up one step at a time clicking on apply each time until you find a screen resolution to your likeing.
if you go too high your monitor may not be able to display the picture at all, if you do this just wait 15secs after hitting apply and the screen will revert back to the last size automatically