Rainbow Loom is wildly popular in America right now - and especially in my house. My son Ryan wanted to BE a Rainbow Loom for Halloween and we figured out how to do it. You can see the rest of my family's Halloween and few more Rainbow Loom photos here.
Take careful measurements of the loom, and the person who will be wearing the costume to calculate proportionate measurements. Use the height to determine a "multiplier" between the loom height and the wearer's height. Our loom measures 10 inches long. We wanted the costume to be roughly 36 inches for Ryan so we used a multiplier of 3.6.