Answer
The bullfighting arena in Colonia had a capacity of 4,200 people. For a small town, it was a major venue, so the Plaza de Toros stands out among the attractions outside the historic center.
The arena was closed in 1912 because of a law on animal protection. Therefore, today it is important not as a venue for ongoing bullfights, but as a historical site associated with another era of entertainment, urban development, and social norms.
On a day trip to Colonia, it is better to plan a visit to the arena separately from the walkable city center. It is not in the compact core with old streets and museums, so you should allow time for getting there and decide how important it is to your itinerary if your day is limited by the ferry.