This depends on a few factors. Additional information is below.
From a video production standpoint - we suggest achieving a 16:9 aspect ratio. This ratio provides the best viewing experience.
From a player standpoint, it can be customized to any size needed and doesn't necessarily have to be 16:9. For example, if a header and footer are needed inside the player, your video is 16:9, but it is possible to set the player to 16:10 to accommodate extra room at the top and bottom. When you create a player - even in our platform UI - you can customize the player ratio. Here's a sample embed code:
<iframe src="https://embed.ustudio.com/embed/D22mmLFC0HVZ/UHmNOGpjHKF7" width="640" height="360" frameborder="0" scrolling="no" allowFullScreen="true" webkitAllowFullScreen="true" mozAllowFullScreen="true"></iframe>
Notice in the embed is the width and height. Before putting it into site code, edit the width and height values as needed. If needing to accomodate a header and footer in a custom player, simply change those values until the video is not covered.