Magento

http://cms.template-help.com/magento_26112/

The above is the link to the template I am using - I am using a custom theme that is designed to show 6 images 3 across and 2 down.  I would like to show 8 or 12 on the frontpage - 4 across and 3 down.  How do I manipulate the size of the boxes that the products are in.  I feel they are too large.  I have looked into the styless.css / .phtml / new.php - I have tried looking everywhere - made a new new.php on the local folder ?  all of this is new to me.  Any help would be great - thank you - J