For what I can see the var is assigned and parsed correctly.
But I will do some extra check.
Vars are assigned in page_header.php and page_tail.php respectively:
$template->set_filenames(array('portal_headerleft' => 'portal_page_headerleft.tpl'));
if (cms_parse_blocks($cms_page_id, !empty($cms_page_id), $cms_wide_blocks, 'headerleft'))
{
$template->assign_var('HEADER_WIDTH', $cms_config_vars['header_width']);
$template->assign_var('PORTAL_HEADERLEFT', cms_assign_var_from_handle($template, 'portal_headerleft'));
}
$template->set_filenames(array('portal_tailright' => 'portal_page_tailright.tpl'));
if (cms_parse_blocks($cms_page_id, !empty($cms_page_id), $cms_wide_blocks, 'tailright'))
{
$template->assign_var('FOOTER_WIDTH', $cms_config_vars['footer_width']);
$template->assign_var('PORTAL_TAILRIGHT', cms_assign_var_from_handle($template, 'portal_tailright'));
}
If you try to insert a DIE just after those declarations you will see that the var is correctly passed:
die($cms_config_vars['header_width']);
Are you sure you don't have something in the column which doesn't allow the column to be sized as you wish? (e.g.: text too wide...)