I suppose that you already checked this out, but I will remind you what the PRS-600 User’s Guide says (PDF version, pages 79 and 81):
“You can adjust the text size using the size menu. Select from 5 different text sizes (S, M, L, XL or XXL). “S” (original size) is set by default.”
“The text size of BBeB, PDF or EPUB files will be magnified as
follows:
- BBeB file: S(100%), M(120%), L(150%), XL(200%), XXL(260%)
- PDF file: S(100%), M(200%), L(300%), XL(450%), XXL(600%)
- EPUB file: S(100%), M(130%), L(180%), XL(230%), XXL(310%)”
Just in case you still didn’t read it. I hope it helps!