Go to Preferences->Common Options and try changing the base font size.
Convert the book and view it with calibre to see if it is satisfactory. If good put it on 650 and see if it is satisfactory
If not go back and make it larger. I believe you use even numbers only, such as 18,20
I need strong glasses to read, but generally I just use the magnify function on the Sony if the text is too small or I have forgotten glasses
Helen