Do somebody know the exact max size allowed for Scratch project to be published?

There is no limit on the overall size of a project - however, there are two other limits on certain things. Exceeding either one will cause your project to not save with the “Project can not save” error:

• You can't have an asset (an asset is a sound or costume) over 10 MB (10,000,000 bytes). If you have a sound that is larger than this, you can use a smaller part of it, split it into chunks less than 10 MB each, or convert it into MP3.
  
• Inside each project is a file called project.json, which contains all of the code, information about everything else in your project, the contents of lists and variables when the project was last saved, etc. This file cannot exceed 5,250,000 bytes (~5.007 MiB, or “5 MB”).
  

You can check the size of project.json at any time by doing the following:

• Download your project, if you haven't already.
  
• Make a copy of the project.
  
• Rename the copy from a .sb2 or .sb3 to a .zip. (.sb2 and .sb3 files are really .zip files renamed to .sb2 or .sb3. If you can't do this, you will need to show file extensions in File Explorer - if you are not using Windows, you will have to look up the procedure for your operating system.)
  
• Open it like you would open any other .zip file.
  
• Sort the files inside alphabetically and scroll to the end, where you should find project.json.
  

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