Resizing of a header/div in a website through css in wordpress [closed]

It’s not possible to give a solution without a link.
In general you would add a max-width value to the div that is your header.
For example let’s imagine that the following code is your header code:

<div id="header">
    <div id="menuHolder">
        HERE WILL BE OTHER CODES SUCH AS YOUR MENU
    </div>
</div>

Then to make your entire header smaller in width you can use this CSS:

#header {
max-width: 800px;
margin: 0 auto;
}

Or you can apply the same CSS to the inner div such as this:

#menuHolder {
max-width: 800px;
margin: 0 auto;
}