Changing the column width for posts (default theme)

I want to increase the width of the text box for the single post page. I changed the “.widecolumn” so that width = 600px and adjusted the fourth margin parameter to 75px in style.css. I thought this would increase the width of the text box whilst keeping it centred, unfortunately WordPress totally ignored these changes. (Yes I did update the file). So does anyone know how I can adjust the column widths and ultimately the width of the full page?

Thanks, Mark.