That is controlled from the functions.php template file. Be careful if you make changes to that though.

I think I figured it out, I haven't bug tested it in other posts and pages
yet, but in the main index it is looking like I want it...

http://troyostrander.com/

To change column width in masonry layout:
in index.php:
change <?php the_post_thumbnail('grid'); ?> to <?php
the_post_thumbnail('medium'); ?>
in style.css:
change the appropriate 280px width to match medium size

I am curious, How does the 280px wide images get generated?

Short Story: How can you change the 280px column width and with that
effect have more or less columns, but still fill the content area
automatically.

Long Story: I tried to change the width in the CSS but the image size
doesn't change and they just overlap. I have also uploaded images
re-sized to the desired width with no effect. I would like to have it
at somewhere between 125 and 200px. But I would need to play with it.
I was drawn to the b+m theme because I want a look similar to tumblr's
archive page. see : http://sometimes-gladness.tumblr.com/archive. (too
bad the animated gifs don't work when being referenced as a thumbnail
in WordPress.)

btw: I am seeing some good and bad examples of the theme by troweling
the forums.....http://www.longwhiteclouddesign.com/ is really nice.
genius! that she made the side bar tab and header the same color!

Best, Troy