Shrinkage on HRT is basically the normal amount of atrophy of the corpus cavernosum (the erectile tissue in the penis) that comes from not having regular erections. This means it'll affect "growers" more than "showers", that it is to be expected more when you take CPA than when you do monotherapy or use bica, that people who feel little impact on their erectile functioning from HRT to begin with are less affected and that finding some workaround to regularly get an erection (such as using cialis or a penis pump) would counteract it as well.

BTW, as the corpus cavernosum is one of the few parts that actually get removed during vaginoplasty, this kind of shrinkage has practically zero effect on how much vaginal depth you can achieve. So people who feel really dysphoric about erections and want to avoid them as much as possible do not need to worry about effects on their surgery results in the future.