My wife and I are going through this right now, we have 2 boys and are trying for a girl. We both have done way to much research to find if there is a way to control that. Based on the information we have found the longer you are probably the more likely you are to have a boy. The reason is y sperm swim faster, but die sooner. So the closer the sperm is deposited to the cervix the more likely it will reach the egg first. So it would make sense then that a shorter penis would mean you are deposting your sperm further from the cervix causing it to swim farther. Causing the Y sperm to die before it can reach the egg.
Now take none of this as gospel, it's just stuff we've found trying to conceive a girl.