However, I managed to achieve roughly what I wanted on this blog by editing my 'template' HTML. With your blog in the Blogger dashboard window goto Template, then Edit HTML. I use the 'Simple' template and changed lines 597 thru 599 as follows:
< min-width: $(content.width);
< max-width: $(content.width);
< _width: $(content.width);
> min-width: 200px;
> max-width: 900px;
> _width: 85%;
Warning: I have no idea how robust this is as I'm by no means an expert on HTML, CSS, or the structuring of blogspot pages. It seems to work for me, especially as I have no side bars. You may also want to experiment with the width adjustment slider on Template --> Customise --> Adjust widths.
This is what we are trying to achieve:
Note thate the wretched Blogger editor window insists on a fixed measure too. Argh!!! Do people not know any typographical principles?
Update (Tuesday 19 June 2012)
I have just discovered that on the Firefox 4 View menu there is a Page Style entry which offers you the choice of No Style and Basic Page Style. The latter, which is the default setting, pays attention to the styling that comes with the web page. But the former throws all the styling away and presents the web page as it might have looked in the 1990s! In particular, you get a variable measure main column. So all I have to do to conveniently comment on a favourite blog is to select No Style. And no need to buy a wider monitor, either. Hurrah!