By "biggest" I mean both in terms of importance and player turnout.
All I could come up with below. Are there any other >100 person vintage events in the US?
NYSE Open
Season: Summer
Location: Long Island, NY
Proxies: 15
2016 Attendance: 157
TMD Waterburry Open
Season: Fall
Location: Waterbury, CT
Proxies: 15
2016 Attendance: 126
Eternal Extravaganza
Season: Winter
Location: Baltimore, MD
Proxies: 15
2016 Attendance: 114
Eternal Weekend
Season: Winter
Location: Columbus, OH
2016 Attendance: 114
Proxies: 0
2016 Attendance: 343
edit: Baltimore EE was missing attendance