How do i customise the width and position of supefish( Nav- Bar ) menu

How do i customise the width and position of supefish( Nav- Bar ) menu

I have a fullwidth menu bar in my website and it's navbar works through the superfish( Nav- Bar ) menu.I have megamenu as submenu of superfish menu. It loads the megamenu content so wide in the screen. It's enlarges the content in the megamenu






I wants to customise the menu width and position in the middle of the screen and main menu.It's shown in below images



Please some one help me to solve this issue and thanks for advance.