Not mostly empty space, entirely empty space. All fundamental particles are point like and have no physical size. Except ..... that's not the right way to meas

Not mostly empty space, entirely empty space. All fundamental particles are point like and have no physical size. Except ..... that's not the right way to measure size, you measure the size of a particle by the forces interacting with it. By that measure an electron has an infinite size (since an electric field is infinite), and a chair has exactly the size it appears to have, because that's the size measured when you interact with it electromagnetically. Objects don't have one single size, their size varies depending on what tool you use to measure them.