Note that this question is as much about physics and engineering as it is about mathematics!
It is a physics question because we are concerned about the "real world", and what mathematical models that adequately correspond to it. e.g. the dot-matrix model breaks down when we start considering length scales around the size of an ink molecule or smaller.
It is an engineering question, because we are concerned with machines actually capable of printing characters, and the ability to distinguish between different characters.
And then once we have adequate descriptions of these, we can consider the mathematical question of how many things are printable, or even dig into more subtle topics such as treating the possibility that different characters might not be fully distinguishable from one another.
