add implementation for osx.

src/Avalonia.Native/WindowImplBase.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Avalonia.Controls;
 using Avalonia.Controls.Platform.Surfaces;
 using Avalonia.Input;
@@ -48,6 +49,11 @@ namespace Avalonia.Native
             Screen = new ScreenImpl(screens);
             _savedLogicalSize = ClientSize;
             _savedScaling = Scaling;
+
+            var monitor = Screen.AllScreens.OrderBy(x => x.PixelDensity)
+                    .FirstOrDefault(m => m.Bounds.Contains(Position));
+
+            Resize(new Size(monitor.WorkingArea.Width * 0.75d, monitor.WorkingArea.Height * 0.7d);
         }
 
         public Size ClientSize