How can I set WPF window size is 25 percent of relative monitor screen

In your MainWindow Constructor add

this.Height = (System.Windows.SystemParameters.PrimaryScreenHeight * 0.25);this.Width = (System.Windows.SystemParameters.PrimaryScreenWidth * 0.25);

Also don't set WindowState="Maximized" in your MainWindows.xaml otherwise it won't work.Hope this helps.

Or using xaml bindings

MainWindow.xaml

<Window x:Class="App.MainWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"        mc:Ignorable="d"        Title="Main Window"         Height="{Binding Source={x:Static SystemParameters.PrimaryScreenHeight}, Converter={StaticResource WindowSizeConverter}, ConverterParameter='0.6'}"         Width="{Binding Source={x:Static SystemParameters.PrimaryScreenWidth}, Converter={StaticResource WindowSizeConverter}, ConverterParameter='0.8'}" >

Resources.xaml

<ResourceDictionary    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"    xmlns:local="clr-namespace:App">    <local:WindowSizeConverter x:Key="WindowSizeConverter"/></ResourceDictionary>

WindowSizeConverter.cs

using System;using System.Globalization;using System.Windows;using System.Windows.Data;namespace App{    public class WindowSizeConverter : IValueConverter    {        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)        {            double size = System.Convert.ToDouble(value) * System.Convert.ToDouble(parameter, CultureInfo.InvariantCulture);            return size.ToString("G0", CultureInfo.InvariantCulture);        }        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) => DependencyProperty.UnsetValue;    }}