Not a true adjustment but unfortunately this is all you can do in safari.
Use this link:
javascript:var%20p=document.getElementsByTagName('*');for(i=0;i%3Cp.length;i++)%7Bif(p%5Bi%5D.style.fontSize)%7Bvar%20s=parseInt(p%5Bi%5D.style.fontSize.replace(%22px%22,%22%22));%7Delse%7Bvar%20s=12;%7Ds+=2;p%5Bi%5D.style.fontSize=s+%22px%22%7D
...copy and paste it as a new bookmark named zoom. Then save the bookmark. Now, for only the current page you are viewing, go to bookmarks and click your zoom bookmark. It will zoom all text 1x. You can keep going bigger by clicking the bookmark again.
It's a PITA and its ridiculous you can't just adjust font size, but that's the sandbox we live in!
Oh, btw, if you reload the page or click the next or back page, it's back to small.
If you're open to trying new browsers, Mercury lets you set a font size in its settings and it saves it until you change it.
It's not as nice as safari (has some quirks), but if your eyesight is letting you not read safari too easily, I'd say it's worth it.