Block widths are set in the file config.php in your theme. Look for the following:

$THEME->block_l_min_width = 180;
$THEME->block_l_max_width = 180;
$THEME->block_r_min_width = 180;
$THEME->block_r_max_width = 180;

/// These values define the min and max width of the left and right
/// sideblocks in the course pages. If not set or false the standard
/// values are taken.