No experience using one in low-vision but it does come with 6 different font sizes that the user can choose from. Size 1 is tiny. Size 2 is what I use and is about the same as in a publication like the WSJ or this forum. The largest size 6 has letters that run from 1/8" high for a small "o" to about 3/16" for a capital "I".
Making a lot of assumptions about your computer,
"This is about the size of the largest font."

I believe I have also read that it is possible to "hack" the Kindle and add different fonts. A check through some of Kindle forums such as www.kindleboards.com may be more helpful.

I have also heard they are on back-order until Feb.