There are 3 ways that the width can be defined. As fixed width, as percentage based or as em based.
In fixed width the width of the blog is expressed in px.
In percentage the width is expressed as a percentage of the parent container (for the blog the outer container is the whole view port or window).
In ems the whole blog flexes as a multiple of the font size.
As to changing the width, we need to consult the html (View Source) and find the outer container - commonly #rap or #page. Then we need to identify the inner areas, commonly #content and #sidebar or similar. Obviously altering one width means you need to adjust the others.
It can be a handy trick to add a border to each so you can see what you are doing during the development.
Remember that even com users can use a mockup of their blog to edit locally. No need for XAMPP etc. Just View Source for the blog, select all, copy into notepad and save as blog.htm. Copy the CSS in the same way. Adjust link to CSS. When you are done paste CSS back into your CS editor at com