Figma Make cannot open in full-screen view

I'm using Figma Make and encountered some problems: 

• noticed there is an option to adjust the preview size, but I can't actually change it. The screen size appears to be fixed, and selecting the option doesn't let me resize the preview or set a custom screen size.
• I can't open the preview in full-screen view.

  

  

   

Is this a known issue, or am I missing a setting?

Thanks!