PowerToys/src/settings-ui/Microsoft.PowerToys.Settings.UI/Styles/_Sizes.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:Microsoft.PowerToys.Settings.UI.Styles">

    <x:Double x:Key="SidePanelWidth">240</x:Double>

    <!-- Breakpoint for wide layout (side panel next to content) -->
    <x:Double x:Key="WideLayoutMinWidth">720</x:Double>
    
    <!-- Breakpoint for small layout (side panel above content) -->
    <x:Double x:Key="SmallLayoutMinWidth">600</x:Double>

    <!-- Column spacing between content and sidepanel -->
    <x:Double x:Key="DefaultColumnSpacing">24</x:Double>

    <!-- Row spacing between content and sidepanel (in small mode) -->
    <x:Double x:Key="DefaultRowSpacing">24</x:Double>

    <!-- MaxWidth of the content panel, similar to W10 Settings -->
    <x:Double x:Key="MaxContentWidth">460</x:Double>

    <!-- Maximum Width of a combo box inside the content panel -->
    <x:Double x:Key="MaxComboBoxWidth">444</x:Double>
</ResourceDictionary>