How to increase scrollbar width in WPF ScrollViewer?

The ScrollBar template reaches out for system parameters to determine its width/height (depending on orientation). Therefore, you can override those parameters:

<ScrollViewer>    <ScrollViewer.Resources>        <sys:Double x:Key="{x:Static SystemParameters.VerticalScrollBarWidthKey}">100</sys:Double>    </ScrollViewer.Resources></ScrollViewer>

Kent's answer can also be applied to easily all scrollbars in your application by placing it in your App.xaml resources, and by specifying the horizontal height key as well.

<Application    xmlns:sys="clr-namespace:System;assembly=mscorlib"    ...>    <Application.Resources>        <sys:Double x:Key="{x:Static SystemParameters.VerticalScrollBarWidthKey}">50</sys:Double>        <sys:Double x:Key="{x:Static SystemParameters.HorizontalScrollBarHeightKey}">50</sys:Double>    </Application.Resources></Application>

Here is a XAML solution:

<Style x:Key="{x:Type ScrollBar}" TargetType="{x:Type ScrollBar}">    <Setter Property="Stylus.IsFlicksEnabled" Value="True" />    <Style.Triggers>        <Trigger Property="Orientation" Value="Horizontal">            <Setter Property="Height" Value="40" />            <Setter Property="MinHeight" Value="40" />        </Trigger>        <Trigger Property="Orientation" Value="Vertical">            <Setter Property="Width" Value="40" />            <Setter Property="MinWidth" Value="40" />        </Trigger>    </Style.Triggers></Style>