How to calculate the amount of space that iCloud Photo Library would take up?
Yes. You can start Photos app and turn off iCloud backed storage. 



You can see that the Photos Library is not my System Photo Library

Then you can import all the photos you want locally and observe how much space it takes. I'm using DaisyDisk, but Finder can show you folder size as well. I have a small system library that I'm using to test out the cloud side of things (15k photos) while I do the heavy lifting on non-system Photo libraries.

Once I'm happy with how albums and such work, I'll probably unify everything so that it fits within 200 GB of space that I'm budgeting for within iCloud.