What a great idea .... you can really personalize it now!
Height from the floor ... while it will come down low, I think I would start with it at about her current shoulder height, so you can start with her current height.
You might want to consider where it will hang .... that may affect some of your decisions as to how you will hang it, how wide it should be, what batting to use, etc.
Being that it is going to be around for a number of years, it will likely need to be washed at some point. Therefore, take that into consideration before you start, in order to prevent shrinkage, colour runs, skewering your markings, etc.
Another thought would be what info do you want on her growth chart? Height is obvious. Date likely. Perhaps weight. I would consider adding embellishments at each stage, personalized to what she is enjoying at that time of her life. Also, I would probably hand embroider all the info, rather than putting on with a marker. Just looks nicer!
Every minute you spend on Pinterest will not be a waste, as with some good forethought and planning, you will create a special memory for you and your DGD.
However .... Keep in mind ... if you start this for this special Miss, then you may be committed to doing the same for any more grands that arrive into your life!
