Well, I’m only a novice myself but if I understand you correctly, you would like one of your posts to display full page and not with the side bar…
When you go to the “edit page” option you should see on the right hand side, something called “Page Attributes” below this is a drop down menu which will say something like “default” just look at the options in the drop down list and “full page” should be in there along with all of the other page options for this theme.