Hi! Welcome to YT!!
Everyone has differen sizing so just going by sizing doesn't always work. THere are many of us here that custom make the clothes to fit. I would suggest getting just a few things while she is still growing and getting her used to wearing clothes. If you wait until she is grown, she won't be used to it and prefer to be nakkie!

If you get the clothing too big, she will not be comfortable and will not enjoy wearing them.
Yorkies have a tendency to be long and thin which makes it hard to buy "off the rack" since most store bought clothes are way too short if you get them to fit their girth. If you get the clothing to fit their length, the little clothes will swallow their bodies!

Just have fun and maybe get her a couple of shirts or dresses to start out.
Good luck! Can't wait to see pix of your baby girl!!