Good point as far as federal law. Up to four units (if owner-occupied) would be exempt. In NJ and NY the exemption is limited to two units if owner occupied, or a room in a single family house even if not owner occupied.
Discrimination is something we all do at all times. Discrimination means to choose or favor. The question is whether the choice is made on the basis of a legally prohibited criterion.
Discrimination on the basis of length of last name (provided there is no correlation with a protected class, in which case the assumption would be that the last name is a pretext and the discrimination is really on the basis of the protected class) would be completely legal in all scenarios.