I can use the '=' and '-' keys on computer to adjust the font size in Google Reader, but can't find how to do it on iPad.
The best way I have found is to use the "Change Font Size" bookmarklet from lucidgreen.net (update: looks like this site is now gone, but the info is directly in this answer).
Its a bit of a pain to setup, but once you do, it works great for google reader (and probably most websites) on ipad.
You'll need to have safari installed on your windows/mac computer. I've tested this with windows 7.
And, you'll need to set itunes to sync bookmarks from safari to the ipad.
1) Copy the following to the clipboard:
2) Run safari and create a new bookmark to any web page
4) Sync your ipad (remember, you must have checked the box in itunes to sync bookmarks)
5) Now, when you go to safari on ipad, you will have a Change font size bookmark.
6) When you're looking at a web page and want the font larger, just choose the bookmark and it will prompt you for a size (defaults to 25px)