My girl is purchasing a road bike pretty soon, but wants to buy used off craigslist or ebay.

My question is; What measurements should I take to determine the basic bike size for her and how does it relate?

I'm 6'1" and ride a 58," but does that simply have to do with my inseam and height?

She's 5'4," normal body length in proportion. What's a typical size bike for a person this tall?