It was my understanding that Ornata is the largest readily available Horned Frog. (Argentine)
According to wikipedia tho, (
Surinam horned frog - Wikipedia, the free encyclopedia) the Suriname is larger and will even prey on Ornata.
"This species was once considered the same species as*Ceratophrys ornata. This dispute was later settled because the Surinam Horned frog inhabits a different habitat than its SMALLER cousin and does not interbreed with it in the wild (but will do so in captivity). This species has been known to prey upon the other species of horned frog, especially the northern race of*Ceratophrys ornata."