What would the bottom measurement of a upside-down cone be if the cone was 30 meters tall?

I am in the process of writing a novel and I am trying to describe the cone-line interior of a large building. I have only the idea of its height, which is roughly 30 meters (100 feet tall). Of course, in my years at school I always struggled with triangular shapes and their measurements, even with help from my teachers. I wanted to know what the surface area of a cone could be if it was that tall? How wide would the place need to be?