step one: Isolate the gene for penis length.

That does assumes that there is a dedicated gene, and that the gene is singular.

It's hardly impossible for it to be governed by multiple genes, like height is.

step three: using CRISPR gene editing, formulate the necessary edits and then bioengineer a retrovirus delivery method.

Only if it's something that can be altered later. Gene editing is not magic that can instantly transform someone if something gets flipped.

It's entirely plausible that the genes responsible might only be active at certain points in time, and after that, altering it makes the cells have different genes, but they won't do much with it.

Similar to how altering the gene responsible for how many fingers you have doesn't cause you to lose or sprout new fingers.