There are a variety of ways to do this. One can use Stylesheets as noted by acl. Perhaps the most direct way is this:
For one Notebook:
SetOptions[EvaluationNotebook[], FontSize -> 16]
For all Notebooks:
SetOptions[$FrontEnd, FontSize -> 16]
You can also set FontSize for different Box types, such as GraphicsBox:
SetOptions[$FrontEnd, GraphicsBoxOptions -> {BaseStyle -> {FontSize -> 15}}]
If you are more comfortable with a GUI, all of these options are available through the Options Inspector in the Format menu.
If you decide to go the advanced route and use style sheets here is a guide to get you started:
David Park's StyleSheet creation notes (.zip file)
Depending on your goals, this question may also be of interest:
How to set default magnification for all windows
sto the end ofOptionfor the first two dealing with the notebooks. – night owl Feb 19 '12 at 00:31