Hello,
You can simply add the style="height: 600px;" Attribute to the #slider Element. Simply change the 600px Value according to what you need your height to be.
Please consider using the following code for the Slider with the Vimeo Video:
<section id="slider" class="force-full-screen full-screen">
<div class="force-full-screen full-screen dark section nopadding nomargin noborder ohidden">
<div class="container clearfix">
<div class="slider-caption slider-caption-center">
<h2 data-animate="fadeInUp">Welcome to Seven Rivers</h2>
<p data-animate="fadeInUp" data-delay="200">A church that's crazy about God, His grace, and making our community beautiful.</p>
</div>
</div>
<div class="video-wrap">
<iframe src="https://player.vimeo.com/video/126060304?autoplay=1" width="640" height="360" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
<div class="video-overlay" style="background-color: rgba(0,0,0,0.25);"></div>
</div>
</div>
</section>
This will definitely work fine. Hope this Helps!
Let us know if we can help you with anything else or if you find any further issues.