Hi Lukas,
It is possible to change the value, although the setting is hidden and you should really contact Support to assist with this. It's not really advised to send messages with large attachments to the spam quarantine, it's not really designed for this purpose and can affect Control Center performance. Is the quarantine managed by admin's only, or do you allow the end users to check there quarantine and release the unscannable messages if desired?
If you allow the end users to release the messages anyway then it may be just as effective to modify the subject line making them aware the message hasn't been scanned.
Kevin