Photo libraries are like downward rolling snowballs; they can get ginormous before you know it. For this reason, Apple Photos lets you clear space on your Mac when you start running out of room.
The one catch is that you may need to start paying for iCloud storage once your library hits 5GB (or however much space you have left after the other data you use). Here's how to optimize your Mac's storage in Apple Photos.
1. Click Photos
2. Select Preferences.
3. Click iCloud.
4. Click Optimize Mac Storage.
Your Mac will intelligently delete photos already stored in the cloud when you begin to run out of space.