I find that 3 sizes are plenty enough. In a low light environment or if my eyes are getting tired I switch to the larger size. I do not use the largest size at all, with the small screen size it is not very useful.
You can also put in embedded fonts for the Sony or replace the default fonts (there is a thread on this around here). You can also change the font size when you create the ebook (provided you are converting it yourself).
I am using the Sony PRS-505 in Hungary without any problems. I cannot use the Connect store, but that is no big deal. There are plenty of other dealers around and tons of free books on the net. If I want to buy a book I go to bookstore and buy it on paper (no DRM, no compatibility issues
).
I am not going to pay for a DRM protected book.