Firstly, Carter, I would like to say that you are a terrible person for giving me such ideas

... but anyway, I went ahead and carried out a variation of your original test as well over the past four days. And the results are pretty similar ... with a fluctuation of up to around an inch in length and about half an inch in girth depending on time/climate/arousal.
For years, I've given the size of my cock according to a quick ruler measurement I did as a-tiny-bit-under-8" x 6.25". I really haven't bothered to remeasure since, as I felt there was unlikely to be any addtional growth after age 23. And the scary thing is that the mean average of the ten readings (taken using the piece-of-string method both for length and girth), despite their fluctuations, could scarcely match those measurements any more precisely: 7.96" x 6.21". So I can go on using my old statistic with impunity. I didn't bother with flaccid measurements, since, well, I found the act of measuring my penis would immediately bring on a semi, at least.
It's been an interesting experiment, in so far as it has made me realize as well that penis length/girth really can vary even more than I thought, and on a daily or even hourly basis.
HOWEVER... I now don't feel the need to measure my penis again ... well, ever, probably!
ERECT LENGTH/GIRTH (
in inches, to one decimal place)
1) 8th Feb morning wood: 7.8 / 6.0
2) 8th Feb afternoon masturbation in bed, shortly before ejaculation: 8.2 / 6.3
3) 9th Feb morning wood: 7.6 / 5.9
4) 9th Feb mid-morning erection after getting back indoors out of the cold following shopping: 8.0 / 6.1
5) 9th Feb late evening masturbation in warm bath, shortly before ejaculation: 8.7 / 6.6
6) 10th Feb morning wood 7.6 / 6.1
7) 10th Feb early evening masturbation on chair watching porn, after becoming erect: 8.1 / 6.3
*
8) 10th Feb early evening masturbation on chair watching porn, shortly before ejaculation: 8.5 / 6.5
*
9) 11th Feb morning wood: 8.0 / 6.1
10) 11th Feb mid-morning erection after getting back indoors out of the cold: 8.2 / 6.2
*measurements 7 and 8 were taken during the same session
OK, way too much information! 