Bigger is always better! Make everything as easy and fun as possible.
Most importantly: know your student! Watch his/her eyes intently as you "read" the words on the page you are showing
them - not yourself (i.e. memorize each page and recite the words as you stare at your student while showing them the words - see if their eyes are really following the words!)
Once you know that they can read a certain font, stick to that font until you need to shrink the words in order to fit more words on the page - and then check your students abilities again (always stare at them as you "read"/recite to them!) Make the fonts bigger if they show any signs of difficulty (or perhaps even difficulties expressed as apparent boredom).
Remember: everyone (you and I included) has a different reading ability depending on the font size (try reading incredibly small, fine print for a while - see how fast and easily you can read it!) - also everyone has different eyes (I need glasses to see far away, but I can read fine print without - however, the bigger the font size the faster I read with less "mistakes" - like when I speed read a font that is too small!)
Make it as easy as possible for them (and yourself) to read - isn't that the point?
Don't forget: I believe
boldness is as important as - or more important than - both font size
and the particular font you use!!
Make everything as
bold as possible! Take up the whole page and use big pages!