Try the fontmenu.default.font.size.list ("17, 18, 21, 25, 31, 36, 60, 86" - looks familiar?) inside com.amazon.ebook.framework.resources.UIResources (in opt/amazon/ebook/lib/framework-api.jar).
I really, really, strongly suggest NOT to modify the original jar files, but to create additional resource bundles instead (as I suggested above). It is much cleaner (and much easier to modify or remove) and has much less potential to shoot yourself in the foot. The link that I posted contains all the required information about how and why this works. If you're wondering about why the system will pick up and use the additional jar file: If the DX is anything like the other models, the cvm startup script builds the class path from all the jar files it finds.
Edit: supposedly, you don't have to change the ReaderResources file at all. I assume that it contains all the *available* (usable) font sizes, while the UIResources one contains the *selectable* ones. Just a guess though, as I didn't spend any time to analyze the actual code logic. This would explain why nothing is happening when you select the "86" entry: the system probably switches to "39" and says "this is the largest font that I have". Or you could try to add all sizes between 20 and 40 to the ReaderResources, so that anything that you select from the UIResources one is "accepted". Just experiment a bit.
Last edited by ixtab; 09-14-2012 at 06:53 PM.