Hi !
I have the slightly bigger 600 and for me it works fine with the smallest font, but I must consider to find a seat on my bus to work with an overhead light. A few times when my bus goes into a tunnel, I have to turn on Medium font.
But I find it really depends on the book and the format they are using. I have bought the latest Dan Brown and Steven Coonts from the Sony store, the Lost Symbol had a much smaller font than the Deep Black. Both ePUB from the same store...
ArcticBoy