Font Sizes on DX
I was not surprised that the DX only has 6 font sizes, because TOPAZ ebooks seem to have these 6 sizes embedded in the ebook as explicit bitmaps. However, the 6 font sizes for MOBI/AZW seem to be about the same size as on the Kindle 1 and 2. This is a disappointment because the larger screen could comfortably support a much larger maximum font size. A simple fix would be for Amazon to add a "large fonts" setting for AZW/MOBI ebooks that doubles (say) the size of each of the 6 font sizes. The reader definitely can support this, since the same Java MOBI reader on the DR1000S has larger font sizes (and more font sizes).
All is not lost, though, because a PDF customized for the Kindle DX can use whatever embedded font you want at whatever size you want. FeedBooks is a good source for examples of ebooks as custom PDFs.