If you want custom sizes between the presets, you must edit the registry. This also controls horizontal/vertical padding.

It is important to distinguish between "Layout" and pure "Icon Size."

Non-integer scaling (e.g., 125%) on a standard 1080p screen. Fix: Use 100% or 150% (multiples of 50% scale best). Or enable "Let Windows try to fix apps so they’re not blurry" in Settings → System → Display → Advanced scaling.