I know this is kind of an old topic, but I really like it, and have a pretty big system with horns to mention.
The largest system with horns in my area (eastern Massachusetts) has got to be the IKEA in Stoughton - it’s around 415,575 square feet!
The system is a large Siemens system (possibly a MXL or FireFinder XLS) from 2005 with ceiling-mounted U-MMT horn/strobes, U-MCS strobes, (I THINK the expansion to the self-serve furniture area has Siemens-branded ZNS horn/strobes) single-action pull stations, and FirePrint smoke/heat detectors.
BTW, should I start an opposite of this topic? (something like Smallest System with Speakers in Your Area?)