The correct size for a gene is the size which will enable the gene to be coded for the particular new faculty. Genes vary in size from a few dozen DNA codons to thousands. You cannot change the size of the gene without destroying the faculty it is coding for or changing it a great deal. That is just one of the problems which evolution has - how to find the correct size for a new faculty. I do not know the answer, scientists do not know the answer, but evolution says that the misterious, almost mystical force "survival of the fittest" knows the answer before there is anything to test for.
Humans and chimps differ in their coding for this molecule by one base pair. That's not coming close to using the slop that's out there.