Not sure I know what you mean by font size is problem in 505.
What is the problem.
I use calibre free program to make epub and sony formats any font size I want. And I have about 4 to 5 zoom levels. In regular mode you have 3 zoom levels but in landscape mode it increases them so that is where i get 4 or 5 font sizes. So batch convert your ebooks to the font you like in the 505 and then at night, with limited light, increase it , and during the day with plenty of light decrease font size. Works for me. The 505 has proved track record on this site. It's the forum room with the most posts and most members (most of the time). The other readers are good too, I was just responding to your comment about limited font sizes. If that is your concern , it shouldn't be.
My font sizes are just the way I like it , and you can make them to your liking too!