Welcome to the world of BC's, saying ouch in a loud voice when she grabs your legs and teaching her to leave it could help you out. I use the command to leave it on many things that I don't want Tucker to do, like herding cats, and other animals, nipping at them or me. It works great on things that he is not to touch. I started by giving him a treat and when he went to get it I'd say leave it and wait. Then when I decided he could have it, I'd say take it. I'd leave the time short between giving it to him at first then worked up the amount of time I would make him wait. As he got that I started using it on other things and say the same command leave it and wait. Another one I have used with him alot is "that will do" when he gets up to or into things, or is playing and I want him to stop I use this command on him.
Border collies are alot of work and you'll have to keep them busy as they get bored very easy and find things to get up to that you might not want her to do. But I have found that nipping (lol) things in the bud is the best way to go, if you don't want her to do it stop it now. Like you said it won't be cute when she gets bigger. My Tucker being a larger male weighs in at about 53lbs. Good luck and get some pic's ready we would love to see them.
A man who looks into a collie's eye to receive an icy stare is but a fool. Be at one with man's best friend and through his eyes you will see his very soul.