I was going by the Dungeon Magazine #149 version of Charon, but CR 24 probably still fits. He compares favorably to the Demonomicon version of Baphomet.
13 feet tall works. Since he is "composed of solid gem and encrusted with gems", maybe based it off a psion-killer (aka crystal golem)? That is 9-1/2 feet tall and weighs around 2,500 pounds, so how much do you think an additional 3-1/2 feet would add?