My friend's mare went lame last spring. She had been using hoof boots for months, but it turned out the sizing was way off. Their farrier found serious rubs and pressure points once the boots came off. They had to switch to a different brand after getting the right measurements.
Years back, I thought getting the right fit for hoof boots was no big deal. Then my own gelding started showing soreness after a few weeks in new boots. Our farrier found they were squeezing the heels, leaving raw spots. That whole mess made me see how easy it is to get the size wrong. Now I pay close attention to the measurements and what the farrier says.