The Host cannot directly control how video frames appear in the live stream. Video frames resize to fit all participants on the screen, both in the Studio and in the live stream feed. Any participant who has their camera on appears in the live stream.
However, participants can turn their cameras off or on to control the size of their video frame in the Studio and the live stream feed.
For example, to make a screen share or media board's video expand to be the largest in the live stream, the participants can temporarily turn their cameras off :
Or to show just one participant's video and a screenshare in the live stream, that participant can enable their camera while the other participants turn their cameras off :