construct.net/en/make-games/manuals/construct-3/plugin-reference/local-storage
Storage quotas
To prevent abuse, most browsers implement a storage quota, which is a maximum amount of data that can be saved locally. On most modern browsers this is defined as a proportion of the free storage space on the device. You can check the available quota on a device by loading Construct and checking the About dialog which shows the quota available. If the quota is exceeded, the On error trigger will fire when writing to storage.
This information is currently not readily available as an expression in the Local Storage plugin (although that could maybe be a pretty simple feature request).
The best we have right now is to simply catch and display the error message when trying to write to storage and there is not enough space available.