What scientific law or principle allows us to
say that all the PE is converted to KE?

The law of conservation of energy

Energy cannot be created or destroyed, but it can be changed from one form to another.

KE + PE = constant

the working equation for the angular size is the product of the physical size and 360 degrees divided by the product of two pi and distance from earth. the minimum size would be 0.559 degrees while the maximum size would be 0.49 degrees.