Does this mean big girl as in English "you're a big girl" or a physically large girl?
Yes, could be either, depending on the context. I definitely recommend saying it to an adult woman you want to like you!
Most often, it means "grown-up girl", i.e. "not a child anymore". But it can refer to physical size. Depends on the context.
I input big girlfriend and it was incorrect. Why?
just holka is not a girlfriend, it is just a girl
a girlfriend could be "moje holka", "tvoje holka", "jeho holka", but it is colloquial
What's the difference between 'holka' and 'd'evc'e' ?