The docs for Chrome extensions chrome.storage.local state:
The maximum amount (in bytes) of data that can be stored in local storage, as measured by the JSON stringification of every value plus every key's length. This value will be ignored if the extension has the unlimitedStorage permission.
However, the docs for Chrome extensions permissions have a note alongside the unlimitedStorage permission which states:
This permission applies only to Web SQL Database and application cache (see issue 58985).
It's unclear to me what this note means in regards to chrome.storage.local, especially considering the issue referenced is from 2010, which predates introduction of chrome.storage by 5 years.