Are you looking for an iPad word processing app for students with low vision? Pages from iTunes allows you to create, edit, and view documents wherever you are. It is possible to change the font size and it is fully accessible. Some students with low vision may prefer to use VoiceOver (built-in screen reader) paired with a Bluetooth keyboard.
Let us know how it works for you and if you have other apps you and your students like to use for word processing.