Movable Type has a number of standard column layouts accessible through the styles menu. This can also be accessed through templates so that you can change layout depending on the content.
What you need to do is set the page_layout at the start of the page like so:
<MTSetVar name="page_layout" value="layout-wt">
the value can be any of the following:
layout-wtt <-- Wide, Thin, Thin layout-twt <-- Thin, Wide, Thin layout-wt <-- Wide, Thin layout-tw <-- Thin, Wide layout-wm <-- Wide, Medium layout-mw <-- Medium, Wide
By placing the code in a conditional statement and using private tags you can change the layout of pages or entries just by setting a tag.