Pages can be set to full width by selecting the “Full Page (No Sidebar)” for the Template option in the page’s Page Attributes metabox.
Posts can’t be set to full width for the content, they will use 2/3 of the page width for the content and the remaining 1/3 for sidebar content. To make posts use the full width for the content area you would need to customise the ‘single.php‘ PHP template.