Welcome to iMore! The only way to increase the size of fonts across the board is to decrease the screen resolution, which carries with it the very undesirable effect of killing clarity and quality of images on your screen. That being said, there are other things you can do to make text bigger in individual applications. Detailed instructions are in the link. It's for Yosemite but it also applies to other recent versions of OS X. https://support.apple.com/kb/PH18831?locale=en_US
Safari settings: As an alternative approach in Safari, go to Safari > Preferences and click the Advanced tab. Under 'Accessibility', select Never use font sizes smaller than and adjust the font size to a comfortable level.