Definitely listen to yzfcathy. On this forum, looking for a bike for a girl, there are plenty of US to get opinions rather than the guys. I am only 5'3", so my opinion will be a little different. BUT, I went from a 1980 Honda 200 standard type bike to a F650GS. It made me nervous, but I took it slow and figured it out. Less than a year later, I rode her to Alaska with my husband, and then went by myself to an off road riding rally type thing. I'm totally hooked, and lets be honest--I ride like a fucking champ.
I think there is nothing wrong with you asking for opinions about what kind of bike to find for your girl. Truly, though, have her sit on a couple different bigger bikes and see what is comfortable, ergonomically speaking. She'll figure out the larger engine.
Btw--Jnich77--Dude, really?. I want to give him the benefit of the doubt, think he is just trying to be funny (and failing miserably), but not sure on that one...
I love my F650GS--my fat bottom girl!
She makes my rockin' world go 'round!
Love is the feeling you get when you like something as much as your motorcycle--Hunter S Thompson (which makes me wonder if I have ever truly been in love...)