NOTE, to clear up potential confusion, this post was updated on 7/11/2011 to reflect more current DNN installations. See the note below.
If you're using XMod Pro 2 or later and your Control Panel pages aren't wide enough, the most likely reason is that you have an invalid DOCTYPE declaration in your page. Many DNN skins have invalid DOCTYPE's. This causes browsers like Internet Explorer to display your site in "Quirks Mode", resulting in inconsistent CSS behavior.
From your browser, view the source. The DOCTYPE appears at the top of the page. Examples of valid DOCTYPE's are below:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
The DOCTYPE is typically defined in your skin's ASCX file. Most of the time we find that most invalid DOCTYPE's are missing the URL portion of the declaration.
UPDATE (07/11/2011):
Most newer versions of DNN allows you to easily change the DOCTYPE on the Host Settings page. See the following post:
http://www.dotnetnuke.com/Community/Blogs/tabid/825/EntryId/2130/The-Fallback-Skin-Doctype-and-skinning.aspx
NOTE: Changing the DOCTYPE can cause some older skins to display differently. Most recent skins should be OK though.