Well this was fun to make. The characters don’t seem to be completely consistent in how they’re drawn, but they’re consistent enough to get a pretty good idea for their relative heights.
Trying to go from relative height to absolute height is more difficult: Steven seems to be about as tall as a doorknob is high, which would put him at three feet tall. This would all be well and good, except Pearl and Greg would have to be 5’1″ tall. We know Greg said he wasn’t that tall, but we don’t think he’s actually that short either. If we assumed Greg to be 5’8″, that would put Steven (and therefore a 10 on this chart) at 3’4″ tall (and mean doorknobs are placed pretty high in Beach City for some reason).
Rose Quartz: 22
Opal: 30 (she likes to stand on her toes a lot though, making her seem taller)
Sugilite has been revised upwards in size since the original posting of this size chart, from 49 to 58.
The images used to represent Steven, Amethyst, Pearl, Garnet, Rose Quartz, and Opal all come from the official art via the Crewniverse tumblr account, while the images for Stevonnie, Sugilite, and Alexandrite come from the Steven Universe wiki (the official art for Sugilite and Alexandrite would have looked awful when stretched that much).