So you have an Unreal 3D project uploaded to the PureWeb Reality Platform and streaming in your customized web client, but the stream doesn't fill the entire available space of the browser window.
In short: you can't. The preview client that is available
The web client template is intended to get you started developing quickly, but naturally you'll want it to fit seamlessly into your existing site.
This is part 2 of the "Web client communication to
This is part 3 of the "Web client communication to