This application (Listen to me), can hold a conversation in a blackboard with a hypo-acoustic person, but you can still read a text relative size.
As it is very light works great on any mobile, but recommended one big screen.

It has a button (Dictate text) that pressing can dictate text then appears on the board, automatically deleting the text.

You can be adjusted within certain limits the size of the text (works in cycles).
It is also possible to type the text to the MTX button then come to the board.
The LP button clears the text typing area.