Capacities (from wikipedia):
Lambeau Field 72,928
Soldier Field 61,500 (will be the smallest in the NFL as of 2008)
FedEx Field 91,704 (largest in NFL)
Michigan Stadium 107,501 (US college football; probably the biggest stadium in the US unless you count race tracks)
McMahon: For special events such as Grey Cup games, temporary bleachers may be built in the facility's end zones; these seats accounted for a record-high 45,010 spectators
[4] at a match between the Calgary Stampeders and the Montréal Alouettes on 29 October 2000.
Commonwealth: The largest crowd in stadium history was the Grey Cup game on November 24 2002 with 62,531 in attendance.
[2]