File Upload Size Limit

I don't remember there being a system-side limit, as Appian can handle any file size that has any realistic chance of being successfully completely uploaded by an end-user before connection interruptions or timeouts occur (i.e. a few gigabytes).

Are you hoping to make a max size?  This is trivially easy to do by way of on-form validations, IMHO.

Nope, just wondering if there was a system wide configuration, and perhaps there should be....

I saw this discussion from a while back on 17.x and some custom.properties setting:

https://community.appian.com/discussions/f/user-interface/11254/issue-with-the-file-upload-if-the-size-exceed-28-5mb/49462#49462

resources.appian.ac.collaboration.MAX_UPLOAD_FILE_SIZE

But I'm guessing this is gone with no collab anymore.

None that I know of.  I'm fuzzy on the back-end mechanics but my theory is that, unlike file extension, Appian has no particular visibility into the size of a file fed into its file upload field until the upload has completed.  At that point it's effectively too late to have a preemptive size limit (in the same vein that the extension requirements work per my limited understanding); and then it may as well be left up to designers to implement a systemwide size validation of their own design, and internally require everyone who uses a file upload field to include that global validation rule within their field validations.  Again just conjecture on my part.

In any case -- I'd suggest if you have a strong use case for a max upload limit (i.e. configured in Admin Console next to the extension whitelist/blacklist), you submit it as a feature request with Appian.

None that I know of.  I'm fuzzy on the back-end mechanics but my theory is that, unlike file extension, Appian has no particular visibility into the size of a file fed into its file upload field until the upload has completed.  At that point it's effectively too late to have a preemptive size limit (in the same vein that the extension requirements work per my limited understanding); and then it may as well be left up to designers to implement a systemwide size validation of their own design, and internally require everyone who uses a file upload field to include that global validation rule within their field validations.  Again just conjecture on my part.

In any case -- I'd suggest if you have a strong use case for a max upload limit (i.e. configured in Admin Console next to the extension whitelist/blacklist), you submit it as a feature request with Appian.