I've sent my third student back to the rental shop for smaller boots this year. When a student has legs that look like rubber, is working hard, and nothing works right, too-big boots are often the problem. Twice the student has had great success after changing to smaller boots. Once the student got boots a half-size smaller which was the same shell size with thicker padding in the liner, and that wasn't much help. (Ski boots are molded in full size increments and half sizes are accomplished by thicker or thinner liners.)
Basic rule...get boots as small as possible without discomfort and buckle them as tighly as possible without discomfort. As always, nothing inside the boot cuff except one pair of medium thickness socks--no long undies, snow cuff, pants cuff, etc. It is OK if the toes lightly touch the end of the boot when the skier is standing in the skiing position (and probably a good thing).