Document per se, it would be challenging to find precise information on the internet and internal resource. (unless someone has actually tried and tested the limits of GAV configuration themselves or have direct contacts to PMs they can probably give this information)
But from from what i can see, as long as the value would be accepted in the threshold, the "Do not scan files larger than: (in MB)" under Protect > Web > General Settings, XG firewall can scan the file.
And you would not want to put up high value in that section as every file that would be downloaded that is within the threshold would be scanned. It would take up resource from your firewall, scan small to medium sized files, as most of the malicious files would come up in small sizes for quicker distribution.
Rapheal is correct for web downloads. It will scan files up the that size, larger ones are allowed without virus scanning. The largest configurable scan size is 1.5GB.
In the policy (you have to edit and go to advanced) you can also configure max download size. So if you want you can say scan up to 1.5GB and block all files larger than 1.5GB.
I do not know the max scan size (or where it is configured) for email.
The vast majority of viruses appear in files under 10MB. Sophos XG default is 30MB.
