Why do you specifically want a big kite?
Older bigger kites will not have as good upwind ability as newer big (but not as big) kites. So unless this is for a weird custom spinnaker style kite for your boat then it's a waste of time.
Get a Core Riot 19XR2LW if you actually want to sail your boat around.
I have 3m, 9m, 10m, 14m and 16m. I was thinking that we may need a larger kite too. I was hoping there was some kites that were fairly efficient up wind such as a Waroo (good enough for me). At least it would be good enough to get going. I suspect that towing a catamaran around will create more line tension and off the shelf kite gear may fail. I prefer to rip up inexpensive kites while the learning curve is steep and potentially costly. It is just for fun too. If it gets more serious then i get more picky about performance.
thank you to all for the suggestions