The following is a good OS disk capacity planning recommended by Microsoft for Windows 2012 and 2016. Even though not all our servers are 32 GB of RAM, we need to plan as such (worse case scenario, so to speak):

Here are some good suggestions to follow when trying to calculate the size of an OS volume.

3x RAM up to 32GB

10-12GB for the base OS depending on roles and features installed

10GB for OS Updates

10GB extra space for miscellaneous files and logs

Any applications that are installed and their requirements. (Exchange, SQL, SharePoint,..)

Taking the full 32GB RAM, a simple OS build would require a drive about 127GM in size.  One may think this is too large for the OS when the minimum disk space requirement is 32GB but let’s break this down a bit…

Why 3x RAM?

If you are using 32GB of RAM and you need to troubleshoot a bug check or hang issue, you will need a page file at least 100MB larger than the amount of RAM as well as space for the memory dump.  Wait, that is just over 2x RAM… There are other log files like the event logs that will grow over time and we may need to collect other logs that will take up GB of space depending on what we are troubleshooting and the verbosity of the data we need.

10GB-12GB for the base OS?

The base OS install size is about 10GB-12GB and that is just for the base files and depends on what roles and features are installed.

10GB for OS Updates?

If you are familiar with the WinSxS directory in the OS for 2008/R2 and up, this folder will grow as the server is updated over the life of the server.  We have made great strides in reducing the space taken up by the WinSxS folder but it still increases over time.

10GB extra space for miscellaneous files and logs?

This may seem to be covered in the 3x RAM but many times people will copy ISO, 3rd party install files or logs, and other things to the server.  It is better to have the space than not to have it.

Source: https://blogs.technet.microsoft.com/askcore/2015/10/05/how-big-should-my-os-drive-be/