Riding boots come in various heights and widths - and vary greatly by manufacturer. I personally have almost 17 inch calves - so custom is required. Ariat boots tend to be skinny calves, Mountain Horse riding boots tend to have larger calves but you still need to look at which "model" is the correct one for DD's calf size.
It also helps to purchase zippers in the boots and/or the elastic gusset to handle slightsizing differences (between the calves, time of day can cause more water weight in the calves more than other times of the day, etc.)
Just really take a look and find a website that tells you how to properly measure her calves, making certain she wears her thickest breeches when you do the measuring.
Alcohlol INside the boot when she gets it on also helps stretch the boot a bit.
Dressage is for Trainers!