How much free space do I need on my Mac to move an app back?
Short answer: At least the app's size plus some headroom, because the app is copied back and verified before the fast-drive copy is removed.
At least the app's own size, plus a little headroom. The app is copied back and checked before the copy on your fast drive is removed, so your Mac needs room for the whole app first.
If space is tight, free some first — for example by moving or offloading a different app — then try again. See freeing up space.