When working with videos in our themes, the theme will automatically resize the videos to be 100% wide, matching the width of its container. This helps the videos to work and resize beautifully for mobile layouts. In situations where you want to control the width, you can easily limit it with a fairly of basic HTML container (div) around your embed code. For example:
<div style="max-width: 600px; margin: 0 auto;"> <!-- insert YouTube embed here --> </div>
Another handy trick when working with embed code in templates like pages or blog articles is to wrap the embed code with an extra div tag to have it resize for all devices including mobile.