Interesting thread.

I have never conducted such a 'study' (i.e., daily measurements), but I've measured my erect and flaccid penis quite a few times.

Although I do have big variations in flaccid length and girth, my erect measurements were quite consistent every time, unlike carter2006 and other posters. When fully hard, the girth is always 5.75", never truly varied from that. The length varies perhaps by half an inch (7.5"-8" - I say that my length is 8" but those are my "best" measurements), but I would not conclude that the size varies, it may just be the accuracy of the measurement (especially since my penis has a banana curve, so I have to use a seamstress tape to measure length).
One thing that was mentioned here is how measuring erect length and girth wth different arousal levels may yield different measurements. I always try to measure when fully hard, so maybe this is why I have less variation.