Congrats on your little girl!
Her birthmark is in the the exact same place as my 17 month olds! My guy's PWS has a "lacey" appearance though.
When he was born, we got the "it's just a birthmark," too. When my son was 2 months old, we switched pediatricians. Our new pedi was immediately concerned and thought he may have KTS. We scheduled an exam with a member of the Vascular Anomolies Clinic in our city. He looked at the birthmark and thought it was CMTC (not a PWS), but sent us for an ultrasound to see what was going on in there. The radiologist didn't see anything that would make him want to order an MRI, so for now KTS has been ruled out.
We then met with the entire Vascular Anomolies team, and the dermatologist diagnosed the birthmark as a PWS, and wanted to start laser treatments right away (he was 6 months old at this time). We have decided to take the wait and see approach as we weren't too crazy about several treatments involving general anesthesia at such a young age. And to be honest I am not completely confidant with the diagnosis (I still think it's CMTC). So far his PWS hasn't changed so we'll just get him checked regularly unless something does change.
I think seeing a pediatric dermatologist is a great place to start. While you are waiting to see the dermatologist, you may just want to take some photos to have a record if there are any changes. Like abatie said, I would leave the measurements to the docs.
Good luck with whatever you decide. It sure isn't an easy decision!