This is always a problem for me too.

Try creating a single-column skin and apply it to the portal as the admin skin. That will deal with it in a lot of places if the problem was that the skin had two columns with one being empty but still taking up width on certain pages. However that doesn't deal with everything, because you will have noticed that some operations such as registration still launch in the portal skin. Registration is one of these.

I have found that I can very easily break a portal by editing the widths of the various controls that make up the registration page. You can find them in the Admin/users folder, but backup before you do anything and be prepared to restore and then recycle your app if it all caves in with an 'xx is ambiguous in the namespace" error. Note the that the reg page is made of about 5 different files for the different sections.

There is one other workaround for this.. Create a page with one column to fit the whole thing then add the User account module to it and after testing it works, set it as the defeault login/reg page.

Rob