Hey,
1) you can use the "Custom Post Template plugin" to select individual templates for your posts/portfolio entries. You can download it here: http://wordpress.org/extend/plugins/custom-post-template/
Then you need to follow the instructions posted here:
http://www.kriesi.at/support/topic/portfolio-fullwidth-but-blog-with-sidebar
2) if you only want to use fullwidth portfolio items:
Change following code in single.php:
<div class="wrapper" id='wrapper_main'>
<div class="center">
to
<?php if( get_post_type() != 'portfolio' ) {?>
<div class="wrapper" id='wrapper_main'>
<div class="center">
<?php }else{ ?>
<div class="wrapper fullwidth" id='wrapper_main'>
<div class="center">
<?php } ?>
and:
<?php
$k_option['showSidebar'] = 'blog';
get_sidebar(); ?>
to:
<?php if( get_post_type() != 'portfolio' ) {
$k_option['showSidebar'] = 'blog';
get_sidebar();
} ?>
You can contact me via email to kriesi2000-dude@yahoo.com if you want to hire me for customization jobs.