So here was the response from BisectHosting.
The 15GB limit is just for the instance, so you wouldn't be able to switch instances if it's over 15GB but you can have the file be as many GBs as needed. If you were to switch instances, you would need to download an archive of the server files to your device if you didn't want to lose all progress when switching.
does this mean it wont shut off if it exceeds 15GB instance?
or rather ... if it grows beyond 15GB
That is correct! You just won't be able to switch the instance if it's greater than 15GB