Hmm, based on the dimensions, there doesn't seem to be a huge difference between the medium or large. BTW, I am looking at the Pandora reference thread -
Givenchy Pandora Reference Thread - Sizes and Measurements.
According to that thread:
Medium Pandora Messenger (shiny version is cheaper than grained version)
9.5" x 15" x 7"
Large Pandora Messenger (shiny)
9" x 15" x 9"
In another thread, Nielnielniel points out that there is a difference in the shoulder straps of the medium and large bags, with the latter having a longer strap. I think you should look at that thread because another tPFer, Peaberry, lists that she is of the same height as you and got the medium. You could reach out to her for advice.
Anyway, here you go -
Do I go Medium or Large Pandora?
And look at this thread for reference pics as well -
Show Pics of your GIVENCHY in Action!!
Good luck!