Hi @DuncanHornby ,
There are no known limitations with file or content size that can result in corruption for the new toolbox format. We have some sample toolboxes that have migrated to the new atbx, and the source code contained in them is in the size of several thousand lines of code.
Validation code is not encrypted. In the future we will make the validation page inaccessible until the encryption password is provided, but this won't be available until Pro 3.1 or later. Your specialized isLicensed code could import an encrypted module within the atbx and call a function to return True/False - this is theoretical I haven't tried it yet.