Hi @wey2go,
I haven't been able to find any official Meraki documentation that specifies file size limitations in regards to AMP or the Threat Grid integration. According to Cisco documentation for the AMP Threat Grid appliance, it states "File size can be 0 bytes (i.e., empty), and no greater than 100MB in size." I can only assume that this would hold true for the MX integrations.
https://www.cisco.com/c/dam/en/us/td/docs/security/amp_threatgrid/amp-threat-grid-appliance-faq.pdf
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