A photo will work. Another way is to compare the font sizes to those from Sony eBook Library
on a Windows PC or a Mac. It always uses a 600x800 pixel window for the text, just like on the Story. So the number of words per line from the same ebook on the Story and in eBL can tell you what sizes are available. The font will look bigger on most PC screens, but the important thing is which font size gives similar words per line on the same ebook.
The E-book Reader Matrix
currently indicates the Story only has 3 font sizes. This is fewer than most readers (if accurate), and a device with 3 font sizes usually does not have a huge maximum font.