After enabling pixel streaming, when running your Unreal game in
Find answers to some of the most frequently asked questions.
If the aspect ratio of your Unity stream is not what you expect, there may be a reason for it, and a way to fix it!
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
What's the difference between publishing 3D applications on the PureWeb Reality platform vs using WebGL?