Geometric Parameter for the assembly

Hi all,
I Create a Bnd_Box and add assembly geometry in to it with BRepBndLib::Add. for the exact measurement, i have set the gap of the bounding box to 0.0.
my assembly geometery is cylindrical in shape. now i am using Get function to retrieve the Xmin,Ymin,Zmin,Xmax,Ymax,Zmax.i am doing (Xmax-Xmin),(Ymax-Ymin),(Zmax-Zmin) for the length,width and height(diameter and height of the cylinder) of the assembly. i am getting height accuratly. length and width of the box (diameter of the cylinder)is more than the actual value.please suggest me the right way to get the diameter of the cylinderical assembly.
Thanks in advance,
Bye
Vikas Kumar Tayal