Page content width

Hi I can find within Raft the ways to change the templates – and find also the layout content width and wide width – however the body text centre justifies – how can I make it the with or the heade and footer like other elements on the page like image sliders? – i would set it the same with as the header or footer – but I have no idea with the px width of those are set to – help please

(its all because I am adding a forum and the shortcode means its tiny and really does need to be the same width as the header / footer)