The Strahov Stadium is not the largest in the world, according to the Czech Wikipedia
The May Day Rungrado Stadium is a 20.7 hectare (2 million square metre) multi-purpose stadium on Rungra Island in Pyongyang, North Korea. It opened on 1 May 1989 and its first major event was the 13th World Youth and Student Festival. It is the second largest stadium in the world in terms of seating capacity (after India’s Narendra Modi Stadium) in terms of 2014 seating capacity and the second largest stadium in the world in terms of its official seating capacity
Source https://en.wikipedia.org/Rungrado_1st_of_May_Stadium
Strahov Stadium opened 90 years ago. Today it is almost unused
The total area of the stadium is 63,000 square metres (6.3 hectares), which is approximately the size of nine football fields. At its peak, between 16,000 and 33,000 practitioners practiced here, which could be watched by 137,000 spectators. The total capacity, for example for a concert, is 220 thousand seats. Comment: nowhere in the world does the capacity of a stadium include spectators standing on the sports field. Besides, the stadium (as a building) did not exist originally. In the days of the All-Sokol gatherings, it was an area with a mound of dirt around it. When the concrete stands were completed, the seating capacity is claimed to be around 65,000.
Source irozhlas.cz/strahovsky-stadion-se-otevrel-pred-90-lety-dnes-je-temer-nevyuzivany