The reason I'm asking is more related to design. Say a client has asked for a fixed width intranet but wants a webpart/list with tons of columns to be displayed. How do you get around not breaking the design?
This question is really a general matter of design, and how you constrain users or authors. The requirement typically states that a "consistent" design will be provided by professional designers, and authors will slot content into that framework. In SharePoint, this is achieved by creating master pages and page layouts (publishing model) that limit what the authors can do in order to achieve this consistency.
Then almost immediately authors, and probably the very people who asked for consistency, will start to try to push at those boundaries. They will want a way of being able to "just" add some arbitrary content somewhere; the title area, say. Or they would like to bend the rules about formatting for some special case. Before you know it the consistent design has gone out of the window.
But really this is just web design - it is not a problem that is specific to SharePoint. If they want a multi-column table embedded in a narrow column there is nothing in SharePoint that will get around the rules of design and HTML. The best suggestion is to use XSLT in an XSLTListViewer to format the list as best you can within the available space. But this is really more a question of HTML design.
If you want to keep all the SharePoint functionality like big list views, you have to compromise the design. A fixed-width layout will never be able to fit in all possible options.
I have had similar requirements and you should go for a compromise: Make the design only a minimal fixed width that expands according to page contents. This can be done with a custom master page and corresponding CSS stylesheets. If you're on SharePoint 2010 you will have to include some dirty table-structures to overcome the ribbon behavior.