This is actually very hard to do, but here's something to try:
First, you need to know roughly the minimum size, in pixels, you would like the left and right columns to be. You can only set the minimum size. The middle column will NOT expand to fill the remaining space, but the idea is that once you set an equal minimum size, the columns should expand at the same rate for bigger screen sizes. So let's assume you've chosen 300 pixels as the minimum size for each of the left and right columns.
Edit the first (topmost) cell of the left column and insert the HTML:
<div style="height: 0px; width: 300px; font-size: 0px"></div>
For the default layout, this is the Rounds column, so the resulting HTML will look like this:
<div style="height: 0px; width: 300px; font-size: 0px"></div>
Round<br><round>
Do the same for the first cell in the right column.
The key is the minimum size has to be as wide or wider than the cell requires to display its entire contents. Any less and the proportionality will not be correct when the window is sized larger. For my test with the default layout (with version 2.6 beta 5, but should be the same for 2.5.12), this was about 170 pixels.