Variable Height Reorderable List Example

    ReorderableList CreateList (SerializedObject obj, SerializedProperty prop)
    {
        ReorderableList list = new ReorderableList (obj, prop, true, true, true, true);

        list.drawHeaderCallback = rect => {
            EditorGUI.LabelField (rect, "Sprites");
        };

        List<float> heights = new List<float> (prop.arraySize);

        list.drawElementCallback = (rect, index, active, focused) => {
            SerializedProperty element = list.serializedProperty.GetArrayElementAtIndex (index);
            Sprite s = (element.objectReferenceValue as Sprite);

            bool foldout = active;
            float height = EditorGUIUtility.singleLineHeight * 1.25f;
            if (foldout) {
                height = EditorGUIUtility.singleLineHeight * 5;
            }

            try {
                heights [index] = height;
            } catch (ArgumentOutOfRangeException e) {
                Debug.LogWarning (e.Message);
            } finally {
                float[] floats = heights.ToArray ();
                Array.Resize (ref floats, prop.arraySize);
                heights = floats.ToList ();
            }

            float margin = height / 10;
            rect.y += margin;
            rect.height = (height / 5) * 4;
            rect.width = rect.width / 2 - margin / 2;

            if (foldout) {
                if (s) {
                    EditorGUI.DrawPreviewTexture (rect, s.texture);
                }
            }
            rect.x += rect.width + margin;
            EditorGUI.ObjectField (rect, element, GUIContent.none);
        };

        list.elementHeightCallback = (index) => {
            Repaint ();
            float height = 0;

            try {
                height = heights [index];
            } catch (ArgumentOutOfRangeException e) {
                Debug.LogWarning (e.Message);
            } finally {
                float[] floats = heights.ToArray ();
                Array.Resize (ref floats, prop.arraySize);
                heights = floats.ToList ();
            }

            return height;
        };

        list.drawElementBackgroundCallback = (rect, index, active, focused) => {
            rect.height = heights [index];
            Texture2D tex = new Texture2D (1, 1);
            tex.SetPixel (0, 0, new Color (0.33f, 0.66f, 1f, 0.66f));
            tex.Apply ();
            if (active)
                GUI.DrawTexture (rect, tex as Texture);
        };

        list.onAddDropdownCallback = (rect, li) => {
            var menu = new GenericMenu ();
            menu.AddItem (new GUIContent ("Add Element"), false, () => {
                serializedObject.Update ();
                li.serializedProperty.arraySize++;
                serializedObject.ApplyModifiedProperties ();
            });

            menu.ShowAsContext ();

            float[] floats = heights.ToArray ();
            Array.Resize (ref floats, prop.arraySize);
            heights = floats.ToList ();
        };

        return list;
    }