rad is 1/2 the diameter. so to go by diameter of circle for a cylinder

do this

tall=10;

size_in_diameter=4.1;

cylinder(r1=size_in_diameter/2,r2=size_in_diameter/2,h=tall,center=true);

if this does not answer your question, then please let us know. Someone will be more than willing to help on this forum.