<Application x:Class="Application"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:system="clr-namespace:System;assembly=mscorlib"
    StartupUri="MainWindow.xaml">
    <Application.Resources>
        <system:Double x:Key="MinWidth">25</system:Double>
 
        <Style TargetType="Button">
            <Setter Property="MinWidth" Value="{StaticResource MinWidth}"/>
        </Style>
    </Application.Resources>
</Application>

Ralf