Xiaomi
Change App Icon Size auf Xiaomi
MIUI ties app icon size to the home screen layout, not to individual apps. That means one slider controls every icon on every home screen page at once, which is why you cannot resize a single icon without affecting the rest of the grid.
To adjust it, go to Tap on Settings ➔ Tap on Home screen ➔ Tap on Icon size. From there, use Swipe right or left to set the icon size to preview the change live, then confirm with Tap on Apply.
What changes on your home screen
Larger icons take up more space per row, so fewer app icons fit on each page and some may shift to the next screen. Smaller icons free up space but can make app names below them harder to read.
If you only want bigger tap targets for a few frequently used apps, a home screen widget or app shortcut is usually a better fix than resizing every icon. Icon size changes also do not affect widget dimensions, so your calendar or weather widget stays the same after applying a new setting.