JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="fixo">
        largura e altura
        relativaaaaaaaaa<br>
        aaaaaaaaaaaaaaaaa <br>
        lnsaçljkdnçada
     </div>
</div>

CSS

body {
    margin:0px;
    padding:0px;
}
.container {
    background: #F00;
    width:100%;
    height:300px;    
}

.fixo {
    background:#000;
    position:fixed;
    color:#FFF;
    top:50%;
    left:50%;
}

JavaScript

jQuery(document).ready(function($) {
    $('.fixo').css({
        'margin-top' : '-' + ($('.fixo').height()/2) + 'px',
        'margin-left' : '-' + ($('.fixo').width()/2) + 'px'
    });
});