If you create a book yourself as an RTF file, you can set the text size as large as you wish. Eg, if you want to read any of the 10s of thousands of "Project Gutenberg" books, there's no issue at all.
If you buy a book from the "Connect" store (not an issue for me, because Sony in their wisdom, only let people in the US do so
) you're stuck with whatever font size the book comes as, plus the small size increase "Reader" will let you do.