I usually do sun's radius to zero for sharp shadows but can't seem to find that "radius" parameter in newer version of 2.8 what did they change it to? thanks:)
The size of the sun is now "angle"
The tooltip describes it as
the angular diameter of the sun as seen from the earth.
Making that value smaller will make the shadows sharper (a more defined umbra), and making it larger will result in softer shadows (or less penumbra).
Do a search online the concepts of Umbra and Penumbra.