There is a quick and dirty way to do this that only requires a simple code change to /modules/YOUR_TEMPLATE/product_listing.php

First you need to set the column widths to a fixed value.
Around line 105 you will find where the column width is calculated based on the columns per row setting.
Below that line, and after the 2nd closing }, add the following:
$col_width = '150';

150 is the width in px that you want your column to be and can be changed accordingly.

Now on line 241, change the '%;' to 'px;'.

Next, you need to set "Product Listing - Columns Per Row" to match your products per page value in order to get rid of any extra linebreaks.

As the window gets smaller the number of boxes that can fit in the centercolumn will get smaller and they will wrap to the next line.

The only other thing I would recommend is adding a fixed height to your centerBoxContentsNew class to avoid misaligned rows.