I've only known one
Dorothea, and she was always called by her full name: not a nn in sight! I quite like the name, but I prefer
Dorothy. The rhythm of
Dorothea, with emphasis on the first and third syllables, doesn't please me: I prefer
Antonia to
Annabella, for instance.
If I needed a nn for
Dorothea, it would not be
Dora.
Thea or even
Dottie would be much nicer. My mother had an older friend whose name was
Deborah; she was always known as
Dora, and it came as quite a surprise when we found out that it wasn't her full name. I couldn't choose between
Debby and
Dora, but I'd prefer
Deborah to either of them.