You could just use HTML instead of a shortcode, which I would recommend instead of layout shortcodes regardless. We include them because people expect them, but I wouldn’t encourage them.
Basically just set the width with a style tag:
<div style="float: left; margin-right: 40px; width: 600px">
Your content here
</div>
<div style="float: left; width: ???px">
Width determined by the size area you have available.
</div>
The widths and margins in the theme are currently set with % so you will have to choose the exact size of the second container based on the layout/columns you’ve selected for the page and how much area is left over.