That would be easily achievable by setting a fixed width on the body
element:-
body {
width: 1200px;
margin 0 auto
}
But I don’t understand why you want this behaviour, it is a very out-dated way of laying out a website. It will only be easily viewable on large screen devices.
I think your time would be better spent improving how the layout adapts to smaller screens, so it looks good at any size.