adjusting my left column to fixed width

I recently found out when viewing my site on a widscreen with a res of 1920px wide, that the site is a variable width, which is no problem accept for the fact that the left column is what stretches so the screen is filled. I found it unusual that this column stretched, instead of the main content area.

How can I set the left column to a fixed width?
I’m using the TPSunrise Theme, which is good except for this.

I know some php and html and I’ve been through my style sheet and have seen the settings, but not sure what to change and to what px. width.
Thanks