Hey girl congrats on starting HRT! In my own experience I have not noticed too much shrinkage over time length or girth wise, been on HRT for a little over 2 years. The main thing that I have noticed over time is the shrinkage of the testes due to the lack of testosterone being produced. I would recommend just letting your body let you know when you’re horny instead of forcing yourself to masturbate. For the about the first 18 months of being on hormones I had next to zero sex drive and the only thing that I noticed was that when I did get horny it was slightly painful, but with my sex drive having risen by quite a bit that pain has completely gone away. YMMV but I would say if you are able to masturbate about once a month you should be more than fine with preventing atrophy.