The solution for your problem is quite simple. First, create a document in TextEdit and insert this text:
Next, in TextEdit click Format > Make Plain Text, then save the file as zoom.css (the name's not really important, just be sure to include the .css extension).
Second, in Safari open the preferences and click the Advanced section. For Style sheet, choose Other… from the pop-up menu, then choose the file you made in TextEdit (either navigate to where you saved it or search for it).
And that's it. If you want, you can change the zoom percentage in the file you make (I used 140% because it's closest to clicking the zoom button twice—it goes to 144%, actually) to make the text larger or smaller. Enjoy.
(Oh, and in case you were curious about the confusion in the first responses you got, the green plus button at the top left of each window is also called the zoom button—unlike in Windows, where it's called Maximize.)
Credit to this post on the Apple support boards for the tip.