How do I make my penis bigger

Im 19 and had an average sized digit about half a year ago but since me and my girlfriend have been having sex with a bit of lube we both think hes got bigger.
Im not sure how using just your hands but maybe a bit of lube might help but dont expect a massive change in the short term as iv always been told that your pretty much stuck with what you got.

Or pull at it a lot, maybe it will adapt