[brucelee]

If you have firmware 4: fbreader (which is already installed) remembers the font size setting. You can long-click a book in your library so that the menu pops up and open the book with fbreader. (check the attached screenshot)

Also if you have firmware 4: You could try coolreader3 and koreader. Those are alternative reading apps which have a global configuration. I use coolreader3 mainly because it can ignore the ebook built-in styles and apply some defined by the user. And that's great when you have many converted or style-less ebooks.

If you have firmware 5: I think you can't do much regarding this problem since you have no "Open with" menu. Most of the books are probably opened with Adobe Viewer which keeps per book configuration. I suggest downgrading to firmware 4. Most of us did that.

Edit:
koreader works with firmware 5 too since it has its own file browser. But it won't be integrated as nice as in firmware 4.