She's probably in between sizes. Both of mine are, it's so fun
You'll probably want more than one style. I have a basket full. It really depends on how much she pulls. Harness vests are good for non-pullers and for leaving on all day. Step-ins are good but they have to fit right. Chattiesmom can make you a custom one. Sammy has short legs so she doesn't do well with step-ins, she wiggles out of everything which is why the vests are better for her. Loki does fine with nylon harnesses. He is bigger (12lbs) so I like the security of the nylon harness - I use a no-pull style. If you do nylon or custom try to get one that is thick enough to be comfy but not SO thick that the buckles/hardware are too bulky.
Premier makes an over-the-head style with two buckles so you don't have to put the front paw through anything. Those are nice. They have the softest nylon. It's called the "sure-fit" harness.
Hope that helps!