Fixed size for editable region

For a website I've made a Dreamweaver template based on a Photoshop slice file
(my first try at this!). In this template I've made an editable region. This
editable region is left-middle of the website, since there's a graphic on the
right side.

However when entering text, the editable region stretches out to hold all the
text. I want the editable region to remain a fixed size (with scrollbars), so
the layout of the website doesn't get messed up. Also I would like some padding
in the editable region.

There's probably a real easy solution to this, but I've been doing searches
all over the web and can't seem to find the answer...

Any help will be much appreciated. A preview of the site is here:
www.kruizerpopweekend.nl/test/programma.html
The other pages show the layout as it should be.

Thanks,
Josephine