Add acess to master previews texture view and add default size for preveiw in uss

com.unity.shadergraph/Editor/Drawing/Inspector/MasterPreviewView.cs

         PreviewRenderData m_PreviewRenderHandle;
         Image m_PreviewTextureView;
 
+        public Image previewTextureView
+        {
+            get { return m_PreviewTextureView; }
+        }
+
         Vector2 m_PreviewScrollPosition;
         ObjectField m_PreviewMeshPicker;
 
+        VisualElement m_Preview;
+
+        public VisualElement preview
+        {
+            get { return m_Preview; }
+        }
+
         List<string> m_DoNotShowPrimitives = new List<string>( new string[] {PrimitiveType.Plane.ToString()});
 
         static Type s_ContextualMenuManipulator = AppDomain.CurrentDomain.GetAssemblies().SelectMany(x => x.GetTypesOrNothing()).FirstOrDefault(t => t.FullName == "UnityEngine.Experimental.UIElements.ContextualMenuManipulator");
             }
             Add(topContainer);
 
-            var middleContainer = new VisualElement {name = "middle"};
+            m_Preview = new VisualElement {name = "middle"};
-                middleContainer.Add(m_PreviewTextureView);
-                middleContainer.AddManipulator(new Scrollable(OnScroll));
+                preview.Add(m_PreviewTextureView);
+                preview.AddManipulator(new Scrollable(OnScroll));
-            Add(middleContainer);
+            Add(preview);
         }
 
         Image CreatePreview(Texture texture)

com.unity.shadergraph/Editor/Resources/Styles/MasterPreviewView.uss

     flex-grow: 1;
     border-width: 10;
     border-color: rgb(200,10,190);
+    width: 200;
+    height: 200;
 }