You can't really average it, because the size depends on the purpose and type. Such as coliseum, football, Baseball, etc...
In my opinion, all stadiums should be 625ft in diameter. Why? Because Miles, feet, and inches all come from ancient Roman measurements. One of them would be a "furlong", which in Latin is "stadium", and measures exactly 625ft.
Just a thought.
The capacity of the Villareal football club stadium is 42,890.
Bill Snyder Family Football Stadium has a seating capacity of 50,000 people.
109,901
The name of RCD Espanyol football club stadium is Cornella-El Prat.
Anfield has a capacity of 45,362.
In Capacity, it's the May Day Stadium in North Korea with a 150,000 capacity.
81,067
According to wikipedia the capacity is 12,435
The Pennsylvania State University's Beaver Stadium. 110,000
The Capacity of St Mary's football stadium is up to 32,689 people.
Sun Devil Stadium has a seating capacity of 56,232 people.
The capacity of Ryan Field Stadium is 47,130. It is the stadium used to host football games for Northwestern University Wildcats in Evanston, Illinois.