For MOBI there are only 3 font sizes and only 3 font choices. The first two attachments are 100 ppi scans of MOBI on the EZ Reader, medium and large. These are low res scans, so they are fuzzier than the actual screen. Some of the other formats allow user provided fonts and have 5 font sizes. The 3rd and 4th attachments are large and extra large for a FB2 ebook. With the latest Hanlin firmware the same display software is used for RTF. I don't have scans of PDF, but it does not reflow but rather zooms in landscape mode. The same is true of some other formats (e.g. ePub).
There is also an alternative Open Source firmware, OpenInkPot, which is still under development but is based on FBReader and so supports a wide range of reflowable formats. It has user installable fonts and last two attached scans are from OpenInkPot using 14 pt and 18 pt Liberation (18 pt is the largest available currently). Assuming OIP is correctly identifying the 14 pt font size, MOBI large (2nd image) is a bit bigger than that.