What is the size of the sun's atmosphere?
The "atmosphere" of the sun is made up of three layers, photosphere, chromosphere, and corona:
The photosphere represents the depth at which we can see no deeper toward the core of the Sun. Think of what a thick cloud looks like when you look down on it from an airplane - it looks solid, but it isn't.
The Sun's atmosphere changes from being transparent to being opaque over a distance of only a few hundred kilometers. This is remarkable given the size of the Sun, and represents such a huge change that we often think of it as a true boundary. When we speak of the size of the Sun, we usually mean the size of the region surrounded by the photosphere.
The gases which extend away from the photosphere make up the chromosphere. These gases are transparent to most visible radiation. The chromosphere is about 2500 km thick.
The chromosphere merges into the outermost region of the Sun's atmosphere, the corona. The corona extends for millions of miles into space above the photosphere.