Antonia is the feminine form of Antonius, but it should be noted that it derives from the family name, not a male given name. Roman women were generally assigned the feminized version of the family name. Thus, Marcus Antonius had two daughters, Antonia Major and Antonia Minor. Likewise for other names -- Julia, etc.
About the Roman naming of daughters... Aside from naming daughters Antonia Major and Antonia Minor, Romans might number them (ie: Antonia Prima, Antonia Secunda, Antonia Tertia...)