Are you asking to change the font size for web sites so that they are a larger size? If so and you use Safari, Safari will remember the larger font size for all the web sites you visit. Just press command and plus until the font size is what you desire. It should remain that way across all sites.
Chrome and other browsers also have the ability to do the same by opening their preferences.
If you need more control than what chscag's suggestions can do for your viewing options, you might want to try using your System Preferences -> Displays pref pane and set your Mac's resolution to a lower level.
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