Is there any minimum requirement of hard disk for Sitecore installation. I understand the database size will depend on the content but if the database is a separate server, for the CMS and CD part is there any recommended value.
I doubt you'll find any recommendation for HDD size because it totally depends on your needs, configuration and needed modules.
However, a basic Sitecore 9 installation is around 2gb without any css, js, images, custom DLLs or configs. You'll have to estimate these for your project.
You can find xDB requirements here