Mertle,
What size have you been riding. If you are able to hold your ground upwind, you must have a reasonable amount of skill.
If your girlfriend is struggling to get moving on the board, then i would suggest a larger board to start off with, perhaps around 140. If you are both going upwind fine, then 132 would be ideal size.
Size will depend on her girlfriends ability, otherwise she wont be enjoying it as much whilst you are travelling
KR