If you can afford it, the iRex iLiad's 8" screen allows larger fonts. If not, I think the Cybook may be your best bet with a 6" screen.
See
Does the new CyBook support ultra-large fonts (>72 points)? (note that I include a 30 pt screenshot from the iLiad in post #13).
For the Kindle's fonts sizes, see
Font Sizes.
For the PRS-700's font sizes, see
text sizes (although these can vary from document to document).
For BeBook (Hanlin) font sizes, see
BeBook font sizes and types.