[Histonet] External diameter of large vein

[Tahseen]

Dear All,
How can the external diameter of large vein like renal vein be measured on 
slide under light microscope?
     As the vein is large and measures several millimeters the micrometer 
scale cannot be used. we also do not want to measure grossly using a venire 
calipre. Our aim is to measure the vertical, oblique and transverse external 
diameter on the slide through microscope. Please also give the reference of 
the method.

Thanks

Dr Robina Shaheen
Abbottabad,
Pakistan