Resize Unity Reorderable List example.

ResizeReorderableListExample.cs

// From MALQUA
// https://feedback.unity3d.com/suggestions/custom-element-size-in-reorderable-list
// http://i.imgur.com/fIbBorr.gifv 

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;
}