The Colosseum and a modern stadium would be about equal. The seating capacity of the Colosseum is variously given as from 50,000 to 70,000 people. Modern stadia have varying capacities that would fall into the above figures.
you can get a stadium but you have to unlock it.
The Diamondbacks play their home games at Chase Field
The Revolution play in Foxboro, shared with the Patriots.
As you did not specify the type of baseball- Pro- amateur , or whatever, the argument has been made that the Fifth Street Ball Park (amateur Only) which has in recent years been used by the Little League, in Hoboken NJ is on surviving ground of the famous Elysian Fields (not the Greek Heaven) where the first ball game was played between the (New York Nine) and the (Knickerbockers) some time in the l840"s. This is well known to Hobokenites. I am familiar with this diamond