(I assume you're talking about the permanent length, not the current one as determined by your arousal.)
It depends on the stage of the game you're in.
In the beginning, sex with cougars and ship passengers, plus getting clients for your girls makes you grow quickly. Later in the game, I think the best source of those needed millimeters is making lots and lots of amateur movies. The longer movies do make you grow your dick more, but this does not increase as quickly as the time spent making a film, so if you're at the game all the time, you're better off making "plain fuck" movies.
