Arousal levels do effect size. Try edging. Since you are by yourself, you bring yourself to the brink of orgasm and stop. Then you do it again. After a few times, you penis will "balloon" to its biggest size possible. Then take your measurement.
How would you measure it when you are with a woman?
You are banging her and then say, "Wait, I have to measure it now!". Take about killing the mood.