If it's on a landing page, you should be able to resize it by dragging it bigger using the drag handles in the lp designer
If it has been taken off the LP and put on some non-marketo page, you'll have to override the options on the DOM node. There are two ways to control it, either the size of the video, or the size of the whole placement, including the share bar at the bottom (if it exists).
videoWidth/videoHeight (controls size of video)
<div class='cf_widgetLoader cf_myvideoid' options='{"videoWidth":1280, "videoHeight": 720}'></div>
outerWidth/outerHeight (controls size of whole widget)
<div class='cf_widgetLoader cf_myvideoid' options='{"outerWidth":1280, "outerHeight": 720}'></div>