How much space for bootcamp? I have around 100GB of space on my (27-inch, Mid 2017) iMac's SSD. I'm planning on using Bootcamp to play some Windows games, and I'm not sure how much space I need to leave for each OS.
I'll be storing all the Windows programs and apps on an external hard drive, so I'm not sure how much I need to leave to Windows 10 itself. I won't be using Windows for much else. I know I have to leave some storage free on the Mac side, but I'm not sure how much.
Windows 10 minimum hard disk space required is 32GB.  You need to start there, add up whatever your games/apps are going to require and allocate that much to the Bootcamp partition.
You get this information by looking up the minimum system requirements for each thing you want to install and adding them up. As a suggestion, most will give two values - the minimum and the recommended.  Always go with the recommended amount.
The key is to research!  I wrote this as more of a “tutorial” of sorts to show future readers how to obtain this info because the next release of Windows, macOS and whatever additional software  will change the specifics. Here’s some links to get you started:

Bootcamp System Requirements
Windows 10 System requirements

Once I have everything accounted for based on the suggested values, I usually add 10% (like a contractor) as cushion. It better to “have and not need than need and not have.”