</code> 
                    and for vertical boxes <code>Heights</code>; for grids we have both <code>Widths</code>
                and <code>Heights</code>. These all obey the same convention:</p>
                <ol>
                    <li>Positive numbers indicate sizes in pixels (similar to "pixel" units)</li>
                    <li>Negative numbers indicate a weighting for variable sizing (similar to "normalized" units)</li>
                </ol>
                <p>By default all sizes are set to -1