The largest size seems to be about 24pt. I enclose screenshots of Word documents at 24, 36 and 48 pt on a page size approximating a 6" screen. These are using the Georgia font. I did not justify the text, because full justification does not work well for this layout.
The Kindle uses a different font, but its largest size is smaller - probably 18pt.
The screenshots will be larger than real life on most desktop screens, but can be printed at 3.6" by 4.8" to get a very close approximation to a generic 6" E-Ink screen.