Add these in css_additional.css template:
If you want to apply the changes to the topic page only (excluding video posts in activity stream or other modules), then add these instead:
Adjust width and height accordingly.
Code:
.videoscontainer .videocontainer, .videoscontainer .videocontainer .restrain { width: 600px; height: 515px; }
Code:
.conversation-content-widget .videoscontainer .videocontainer, .conversation-content-widget .videoscontainer .videocontainer .restrain { width: 600px; height: 515px; }