From the experts:
Antonia is stronger than most feminized boys’ names, reflecting the pioneer spirit of Willa Cather's classic novel My Antonia. Antonia is hovering near the bottom of the US popularity list, which may be an excellent reason for you to use it.
Antonia was borne by several prominent women in ancient Rome, including the mother of the Emperor Claudius.
A recent distinguished namesake is British historian/biographer Lady Antonia Fraser. It is also the full name of actress Nia Vardalos.
Antonella is a pretty feminization; Antoinette, not so much.