Bigger than foot. I'd say 12-14 inches, maybe 16 inches. And they get there fast, too. Whatever you think you eventually MAY want to live in your tank with your adult Zonatus, put it in about 6 months before you get your new baby Zonatus.
Because a Zonatus will outgrow every other Vieja I've ever had, and you end up with a 8 inch Zonatus whipping a 5 inch syn/Argentea/Blackbelt/Bifa. That's what happened to me. Here's a video.
Yea, I'm in Lafayette. I got that Zonatus from Gage at Cichlid Connection a couple years back. He had to go, though. He was too crazy, too much bigger than the rest of the fish in the tank. I have 4 smaller ones now that I got from CoA. They're kinda crazy, too, but at least they're smaller than the rest of the tank.