Avoid small fonts, they're bad for your eyes.
Because today almost all reading is done on a computer, you can set a larger font no problem. Plus, computers make lookups and sentence mining easier.
Some printed books are written in small fonts, but you don't need to practice reading at smaller font sizes in order to get used to them. People who are fluent in reading Japanese percieve kanji as single units without paying attention to individual strokes. Therefore, if you can read Japanese in a large font fluently, you're going to be able to read in smaller fonts too if you have to.