Add fixed aspect ratio and initial size to PreviewNode

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Nodes/PreviewNode.cs

 
         public void SetDimensions(float width, float height)
         {
-            m_Width = width;
-            m_Height = height;
+            float newSize = Mathf.Clamp(Mathf.Min(width, height), 150f, 1000f);
+
+            m_Width = newSize;
+            m_Height = newSize;
         }
 
         public float width
         public PreviewNode()
         {
             name = "Preview";
+
+            m_Width = 208f;
+            m_Height = 208f;
         }
 
         protected override MethodInfo GetFunctionToConvert()

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/MaterialNodeView.cs

             m_Preview = previewSystem.GetPreview(inNode);
             m_Preview.onPreviewChanged += OnPreviewChanged;
 
-            node.onReplaced += OnReplaced; 
+            node.onReplaced += OnReplaced;
 
             // From OnDataChange()
 
 
         void OnResize(Vector2 deltaSize)
         {
-            float updatedWidth = Mathf.Min(leftContainer.layout.width + deltaSize.x, 1000f);
+            float updatedWidth = leftContainer.layout.width + deltaSize.x;
             float updatedHeight = m_PreviewImage.layout.height + deltaSize.y;
 
             PreviewNode previewNode = node as PreviewNode;
 
         void UpdatePreviewTexture(Texture previewTexture)
         {
-            if (previewTexture == null) 
+            if (previewTexture == null)
             {
                 m_PreviewImage.visible = false;
                 m_PreviewImage.RemoveFromClassList("visible");