I edit/butcher my OS and with all the software installed disk C uses up about 9-10GB, which means my OS partition has a lot of free space I'd rather allocate to my Games partition. How can I change partition install size requirements? Windows 10 default requirement is some 30GB!
use disk management to shrink the partition with 3th party app make the partition of the game move back and expending to the free space that has been freed
If you're referring to a clean Windows installation, use the Microsoft-Windows-Setup component in the WindowsPE Settings pass (autounattended.xml) using Windows System Image Manager. Then you can configure your disks however way you like using DiskConfiguration. If you're referring to a live system, programs such as MiniTool Partition Wizard and AOMEI Partition Assistant will do what you're asking in their free versions, though I find MiniTool to have more options in their free version than AOMEI. From there you can resize, create, add partition identifiers if you want, etc. If it's simply just shrinking your main Windows drive down to make another partition for installations, as lite8 said, you can easily do that with Disk Management.
I use Easeus Partition Master 9.x and that just works great. But, MS Disk Management using for to partition resizing, will just resize to 50% of the HDD size, not really custom size below 50%! You should also keep in mind to have the system partition big enough, at least 50GB, because some extra space is needed for various operations, the Page file etc., etc., besides of the needed temporary space need for momentum use! Anyway, it's all a personal matter what the user need or like!