Petra is deffinately a feminine name and it's so sweet and pretty to me.
Petronella is also nice but is a little bit long and unheard of in the USA. If you like
Petra better I would deffinately use that or you could name her
Petronella and use
Petra as the name she's generally called or something else like
Nella,
Ella,
Petro, or
Ronnie as a nickname.
I hope this helps.
~
Laurie :)