You can specify the width for each cell of hbox with the widths property as follows.

<hbox width="100%" widths="10%,20%,30%,40%"><label value="10%"/><label value="20%"/><label value="30%"/><label value="40%"/></hbox>                

The value is a list of widths separated by comma. If any value is missed, no width is generated for the corresponding cell and the real width is up to the browser.

Similarly, you can specify the heights for each cell of vbox with the heights property. Actually, these two properties are the same since the orientation of a box can be horizontal or vertical depending on the orient property.