Make all content larger: Choose Safari > Settings for [website] and choose an option from the Page Zoom pop-up menu, or choose View > Zoom In. You can also pinch open on a trackpad that supports gestures. Safari remembers the zoom level when you return to the website (unless you pinch open to zoom).
Make only the text larger: Press Option while you choose View > Make Text Bigger. Safari remembers the text size when you return to the website.