So, how do you measure length?

[UrsusMinor]

I know that sounds like a dumb question, but when people talk about the loss of penile length, how is it being measured?

If it is measured with a ruler from the base to the tip, then you get a shorter measurement than if you lay a tape measure down the shaft from the base to the tip.

In other words, if it were an archery bow, with a ruler you are measuring the bowstring. With a tape measure, you are measuring the bow.

What exactly are people reporting here when they report changes in length?

[NeoV]

It should be measured from the top, from the public bone, never the side.

After that, weather you use a ruler or a tape measurer or string is up to you.
To be fair, one should use measurement tape since it's flexible, but a ruler will definitely show you what you've lost as far as distance reached.

[LWillisjr]

And you must be consistent with either erection length, or stretched flaccid length.