Do not stay up and run small.
I wore these tights for an entire day during a 2-show performance. I had them on for approximately 12 hours. Several times throughout the day I had to pull them up because the crotch was slipping down. I am 5'1" and weigh 108. I had to wear a size 'B' because the 'A' was too small. I would not buy these again. I am, however, impressed with their durability. They did not snag or tear.