Plane Angle
The plane angle a is determined by the length of the arc a and the radius r. The biggest plane angle obtains for a full circle (2πr) over 2π.
Solid Angle
Analogously, the solid angle Ω describes the size of a sector in three-dimensional space. It is the product of area A and the square of radius r². The biggest solid angle obtains for a spherical surface (4πr²) over 4π.