it is possible to preview the radius size in km when drawing the circle
No.
is there any other way to define a radius with referencethanks
Not sure what you mean by "with reference".
I found a solution for thiscreate a defined radius in KML format and then convert to GPXhttp://kml4earth.appspot.com/circlegen.html
it is possible to preview the radius size in km when drawing the circle