When there is a requirement to alter the out of box attachment size and need to customise anything related to attachments, please take a note of the OOB values and system behaviour, as follows:
The default attachment size system accepts when 'com.glide.attachment.max_size' property is empty is 1024MB.
The maximum attachment size we can allow in the system, so that the instance would not run into performance issue is no specific value or a hardcoded value.
It can be set based on business requirement to a higher value and can be tested out on the instance to confirm there is no performance impact.
ServiceNow suggests to keep it at the OOB default value and test the behaviour on any sub prod instances before changing the values on the production instance.
To monitor the progress of the attachment being uploaded, there is a status bar present on the bottom left corner of the screen depending on the browser being used, for example the chrome browser has it, while uploading any data or attachment in ServiceNow.Please check for the status bar accordingly.