Here’s the scientific approach: (a) Remove the penis using a sharp instrument such as a scalpel or a fine-edged hunting knife. (b) Attach the severed penis to a pump (water is good but compressed air will work too) and inflate the penis to its maximum capacity. © Take a string and proceeding from the base, trace out the line to the tip of the penis. (d) Remove string and place against a ruler. (e) Take a tape measure and determine the girth in three places: base, mid-point, and widest part of the glans. You will now have an accurate set of measurements.
Some useful advice: Firstly, report the results to your significant other in centimetres rather than inches because this sounds more impressive and will boost his ego (which may need significant help given your prior removal of his pride & joy). Secondly, when attempting to re-attach the penis after measurements have been completed, there’s no need to employ an overlock stitch. A regular running stitch will do just fine provided you’ve used enough SuperGlue.
I hope this was helpful.