Whether you go with knee high or thigh high is really up to you. If you are getting varicose veins in your thighs as well, youd want to go with the thigh high or even waist high, but if its only in the lower leg you can do knee high. Since they are just starting out I'd say go with the 15-20 mmHg. If you decide you want even more support later on, you can step up to the 20-30 mmHg.