Increase text size on iOS Safari
New Answer: Base on this and this


Bookmark this page
Navigate to your bookmarks and edit the bookmark added in Step 1
Change the name to "Increase Text Size"
Replace the URL with the code labeled "Increase Text Size" below
Repeat steps 1-3, but change the name to "Decrease Text Size"
Repeat step 4 but with the code labeled "Decrease Text Size" below


Increase Text Size: 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

Decrease Text Size
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



Old Answer:

From this article


  Here’s how you can use Reader in Safari for iPhone or iPad to increase
  text size of web pages:
  
  
  Open Safari and go to any webpage with a lot of text, like an article or news piece
  From any Safari browser window in iOS, hit the “Reader” button in the URL link bar to enter into Reader View – the Reader button looks
  like a series of lines atop one another
  Once in Reader Mode, tap on the “aA” icon in the corner of the screen
  Now tap the larger “A” button on the right side of the popup menu to increase text size
  Tap the “A” button repeatedly to dramatically increase the font size of web pages in Safari Reader mode
  


Although this does not change the text size, you can pinch and zoom.

Lastly, you can change the font size. Although this will not help with Safari it will increase text size in other apps.