First a note since you mention Bootstrap – if you are using the Bootstrap add-on the menus will use the Bootstrap theme (whatever it is). The “sm-blue” theme (class) is not supposed to be used in combination with the Bootstrap add-on.
If you are not using the Bootrap add-on, you can center the items of a horizontal main menu with some additional CSS – a sample code is posted in the following discussion:
As for taking the full width – a horizontal main menu would take the full width of its container by default since we have the following styles by default on the demo page:
So it depends in what container you would put it and if it would have any margin/border/padding applied. Or you could also change its positioning to something like (which would work on the demo page with the “sm-blue” theme but might not work with some specific layout, of course):