HTML Weird Lineup Prob !

[alex_read]

Hi all, I've got the following that I'm stumped on. Any help which can be given to help me understand why this is happening would be appriciated.

I've got a page(pic attatched) with my select listbox down the left of this page. This listbox is in a table, and I've used :

<TD Rowspan="3">
    <Select Multiple style="height:310px; width:65%">
    <Option>A</Option> ...
    <Option>Z</Option>
    </Select>
</TD>

To make it spread accross 3 rows of the table (the attached pic shows the table outline). Unfortunatly, I've had to set the height in pixels to take up the full height of these rows. I can use a percentage figure with the width, but as soon as I try this with the height, none of the listbox shows ?!??!?!
Any ideas why this would happen Thanks!