Not sure how you measure, but I don't think it should get longer, mine doesn't.. It really is quite simply, measure the same way everytime, and you will be fine. Most people try to measure by straightening it out parallel to the floor.
my problem is i can not get a 100% rock hard erection for the last few months, so i dont know my exact measurements. i measure when i have a rare morning hard on or am talking to a chick on the phone about sex and i get horny. i stand up and measure it before it disappears.
also, do you guys base your measurements off of your size pre exercise, or post? i do it pre.
I think nearly everyone does it pre-exercise. Or it would be "cheating". I even hold off measuring until my on day after being off. That way there has been more than 24 hours since I did any form for PE. But that is just me.