I believe ya man. It can be difficult to sustain a hard-on while your concentrating on positioning the ruler just so. Also hand size is not really an indicator of size, because there's a lot of variation between people. If you really want to convince people, you should use a standardized product, like an iPod or a water bottle. I would also suggest that you use a webcam so you don't have to fumble with a camera/phone. Try to get the entire ruler in the shot too.
As for your other comment about making your dick bigger, I'm curious about why you want to. I'm doing some jelqing
routines and their at least making my erections stronger. But for me it's more of a curiosity thing.