1. Go to About Phone > Software Info and tap the Build number 7ish times to unlock the developer options menu.
2. Back out to settings, enter the Developer options menu now at the bottom of the list.
3. Scroll down until you find the Minimum width setting in the Drawing section about 3/4 of the way down. It's set to 411dp by default on the S8+.
4. Change it to whatever you prefer! Higher number = everything gets smaller, Lower number = everything gets bigger.
Enjoy your extra space!