I see that Blogger will let you set the blog width under Overview-> Template Designer->Adjust Widths.
However, this seems to be a fixe width - is there an easy way to set a max and a min width?
You may want to set a fluid width, an example blogspot blog is here http://vinfirst.blogspot.com/ and here http://bloggeruniversity.blogspot.com/2006/08/fluid-3-column-blogger-template.html
Google for "blogspot fluid width" or "blogspot width percentage"