When would my sperm stop becoming fertile when I'm on HRT?

I'm considering going on HRT, but I what to know after how many months will my sperm cease to be able to impregnate a woman. I also want to know how much my penis will decrease in size and in how much per month. Also after how many months will I not be able maintain an erection? I have penis six inches long when it's fully erect. Any help on this is tremendous for me and my gf.