From Test-Scratch-Wiki

Translate: - English 
Main article: Resolution
No, it cannot be changed. However, the project can be shown in a larger space full screen mode. When using vector graphics, this will not reduce the quality or cause pixelation. The Scratch Team is also considering a 16:9 option for Scratch 3.0.[1]

See Also

References

  1. https://scratch.mit.edu/discuss/topic/199574/?page=25#post-2090971