"The stadium is very big."
Is 会場 more like an assembly hall? When I looked up stadium, the closest thing I got was 競技場 (きょう ぎ じょう) - Competitive skills arena. I feel like that's a little closer.
会場はとても大きいです。This is a basic sentence with almost no possibility of multiple strange translations. What could possibly go wrong?
How do I get it to accept スタジアム? Fairly sure that's far more commonly used to refer to a sports arena than かいじょう (which is a pretty generic term).