If the three presets aren't quite right, you can manually "zoom" your icons to any size using your mouse wheel. Windows 11 supports through this method. Customize the Desktop Icons in Windows - Microsoft Support

In the right-hand pane, look for two string values:

With the grid disabled, you can manually drag icons as close together or as far apart as you like, regardless of their size. The downside is that icons won't snap to a neat grid, which can look messy.

<TextBlock x:Name="IconSizeLabel" Text="Icon Size" />