I am 12 and my penis is 4.9 inches. How long do you think it will be
fully grown? My dad's is about 5.6 inches. Do you think I will ever be that
big or bigger?
It is impossible for say from the information you gave. To illustrate, suppose you walk into a room and see a 3 inch candle burning and I ask you how long will that candle continue to burn. With only the information I just told you, there is no way to know. I have no idea what stage of development you are in, though I know you are past stage 2
. I can be fairly confident that you haven't finished growing, so I can assume your penis will be getting bigger. But, I can't tell you how much growth you have left. While knowing your dad's size gives you a rough idea about the genes you inherited from his side, that is only half of your genes. You can be taller or shorter than he. Your penis could be larger or smaller. You won't know until you finish growing. See: When does a penis stop growing?
for additional information.