ecostin has discovered a neat hack that allows you to set custom font sizes on Kindles using 5.6.1.0.5 without jailbreaking. It's amazingly simple: create plaintext file with 8 numbers in it, connect Kindle to computer via USB, copy file into root directory.
It works great on my Kindle Voyage.
Details here:
https://www.mobileread.com/forums/sho...93#post3060893