How big is the biggest hibiscus ever recorded?

Texas Giant Hibiscus is a giant rose mallow which has the largest flowers of any hardy perennial. These are descendants of the native hibiscus found in Louisiana and other Gulf South states. Initially, they were sold as The Mallow Marvels.
Named after Ying Doon Moy, research and development horticulturist at the San Antonio Botanical Center, who cross-bred a Hibiscus moscheutos hybrid with Hibiscus grandiflorus to create the largest, hibiscus in the world. Moy Grande has 12 inch blooms and are among the most spectacular and easily grown plants. Moy Grande recently won award status from the American Horticultural Society.
See http://www.angelfire.com/journal2/flowers/pcd19.html for examples of other giant flowers.