Margins become smaller

Margins become smaller

I want to make left and right margins on my webpage. I make a main div
element and set margins <div id = "wrap" style="display: block;
margin-left:200px;margin-right:200px;"> ... </div> . But when I resize the
page in a browser, margins become smaller. However, I need them to be
fixed,so if the content size+left margin+ right margin becomes larger than
the screen,the scroll bar must appear.