Is your Sony the 505 or 700? I ask, because the thread "very detailed instructions for changing the default font..." is a great tool. It makes use of PRSCustomizer. I have very poor eyesight also, so I've changed my font to ariel. At the L setting, it is nice and big and bold. In landscape mode, L is too big but M is just right.
If I am re-formatting something that is not LRF, such as ADE, I find that changing the font to ariel and 22 pt font shows up great when I put it onto my 505. Increase the line spacing to 1.25 to increase the contrast between letters and background.
Both of these options give me letters that are about 7 mm. Anything bigger seems to take up more line space and I end up turning pages too frequently. People with normal eyes can probably read it from across the room!
For most of her career, my sister has worked in accounting offices. Tax season was that special time of year when we never saw or heard from her. She worked from sunrise to way past sunset, every year. Then on 4/16 each year, they would close the office for a week and we would go to the beach.
Good luck and let us know what you discover!