In SharePoint you can specify two different kind of master pages:
- Site Master Page (applied to publishing pages)
- System Master (list views)
So you can use two different master pages one with fixed width for the publishing pages (Site Master) and one that use the full width that will be applied to the lists (System Master). This is the normal approach how I do it.
The other thing is that the you can also change page layout to be fixed with too. The approach for that is that you reference a css in the additional page header section in the page layout. The code below is all you need to do to have a fixed width layout in master or page layout (suitable for SharePoint 2013).
/*** Default setup ***/
/* background-color of the workspace */
/* defines 960px by using 60rem = 60 * 16px (Default Font Size) */
/* background color of the content */
/*** Fix for the dialogs ***/
/*** clear fix ***/
The clear fix need to be inserted in a specific place in the master page. You will find this on the SharePoint 2013 blog post.
This code also maintains the width of the modal dialog so nothing to worry about. You will also find some addition information on that in two of my blog posts for Fixed width for SharePoint 2010 - the fast way and Fixed width for SharePoint 2013 - the fast way
What you should consider when you plan your design is that it will distract the user when you switch between fixed with and full width. It also depend on the use case you have. If it's an intranet project you should stay with full width over fixed width.
If you are worried about the content that the lines gonna get to long to read then you can limit the content to be fixed width. This is the possible best ways to do it and improves the readability of the content.
The full fixed width approach is mainly suitable for public facing web sites where you want to limit the user to a specified screen size.