girth at the maximum point of thickness wheter it is at the base,middle shaft or head... and at middle point too for additional info.. length is taken when fully hard with the erected cock held parallel to floor and the measuring tape starting from where the cock meets the body on the TOP SIDE with a slight pressure applied to compress the fat pad on the pubic bone... another good measurement is bone pressed flaccid stretched length... a very reliable one and is taken stretching to the max your flaccid cock (holding it just below the glans) and always holding it parallel to the floor...this time you have to press the ruler till the pubic bone and measure from here to the tip of glans...
Also is important to know your soft sizes... measure your length with that cock just hanging form the point where it meets your body and the tip of glans... and the girth is taken with a tailor tape at the maximum point along the shaft (and for additional info at the head) with a very slight compression of the skin...
I hope I have been clear ;-)