Stretching the penis can increase size
Stretching consists of attaching a penis stretcher or “extender” device to the penis for set periods of time. The device exerts a constant traction on the penis, which, in theory, lengthens and widens the penis. The traction supposedly causes the cells in the penis to “split” and then reform and expand. In 2002 a study published in the International Journal of Impotence Research concluded that penis extender devices are effective at elongating the penis and furthermore the increases in length correlate with the amount of time the device is worn.  A similar study conducted by Paolo Gontero of the University of Turin was published in 2009 in the British Journal of Urology. 
Taken from Wiki
Link to the Study Conducted by Paolo Gontero
In 2006 researchers succeeded in replacing a rabbit’s penis with one grown in a laboratory. The penises were grown on a matrix from the rabbit’s own cells, and contained nerves and blood vessels allowing them to become fully functional.
From wiki also.