Links to Relevant Wiki Articles(s):
Here's a breakdown of the various size limitations for what can be uploaded ir saved on the Scratch website:
There is no limit to the total project size. HOWEVER...
The size limit for individual assets (sound or image) is 10 MB.
The project.json size limit is 5 MB.
The Scratch app versions that you download to your computer, tablet, or Chromebook do not have any explicit limits; they are constrained by the practical limit of the available memory and hard drive space on your computer.
If you're concerned about hitting the size limit and you're comfortable with .zip files, you can check your project's approximate size by:
Downloading the project. The file will end with .sb3
Renaming, replacing the .sb3 with .zip
Unzipping it
Opening the folder and checking the size of each file inside (Image files, sound files, project.json).
To reduce the size of your project, you can try to shorten long lists, remove duplicate sprites and use clones instead, replace images with lower resolution versions, and cut sounds up into smaller pieces so none are larger than the size limit.