(How big is yours?)
Derived originally from the popular anime, FLCL (Furi Kuri), the term refers to the status of your maturity. Those who behave like children are more likely to have smaller guitars than adults. It is also relative to age; being that immature adults will have very small guitars, while children who have been forced to mature in response to those around them, will have very impressive instruments.
Sometimes refered to as your 'bat', one would be excused for thinking in the style of Freud that the guitar was a phallic object. But, females possess guitars as well, and can wield them just as well as the male.