There are only a couple of those questions that I can answer, but I will at least answer them for ya.
You measure for a rug or blanket with a soft measuring tape. Start at the center of his chest, go around the widest part of his body, and to the center of his rear end. The number in inches will be the size of his blanket. Blankets are sized in even numbers, so if he measures 79", you would order an 80.
If by medicine boots, you are talking about Professional Choice Sport Medicine Boots, they are protective and supportive leg wear for horses. They actually help support the horse's tendons and ligaments in his legs. They actually have a size chart on their website. They can be measured for by measuring the circumference of the lower leg, and the height from fetlock to knee or hock, I believe. Their chart will tell you where and how to measure, and what measurements are for what sizes.
As for the bell boots, their purpose is to protect the foot in case of an overlap, so it helps protect more is they are closer to the ground. I would be willing to bet that with your girl's size she will require a large. I would say the same for the medicine boots. Her legs may look skinny on her, but chances are they are bigger than a medium boot is meant to fit.