[CustomPropertyDrawer(typeof(TestSprite))]
public class TestSpriteDrawer : PropertyDrawer
{
    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        return 140f;
    }

    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        var spriteProp = property.FindPropertyRelative("mySpriteTest");

        EditorGUI.LabelField(new Rect(position.x, position.y, position.width, 16), "Sprite");

        Rect objectFieldRect = new Rect(position.x, position.y + 20, 128, 128);

        EditorGUI.PropertyField(objectFieldRect, spriteProp, GUIContent.none);
    }
}