Use the foundation's in-built grid classes large-10 small-10 medium-10
<div id="myModal" class="reveal-modal large-10 small-10 medium-10 columns" data-reveal>
<h2>Popup title.</h2>
<p class="lead">Your couch. It is mine.</p>
<p>Im a cool paragraph that lives inside of an even cooler modal. Wins</p>
<a class="close-reveal-modal">×</a>
</div>
And you have to remove this line of css :
.reveal-modal {
width: 100vw; //remove this line from your css
}