Theoretically, there is no limitation on email size to be fetched into the helpdesk. The email size would depend upon the resources available on the server hosting the helpdesk. Further, the helpdesk has following settings which affects email file size:
Email Size Limit (Download): This defines the maximum size of email that can be fetched into the helpdesk, the helpdesk rejects any message which is larger than this size (in kilobytes). By default, the value of Email Size Limit is set to 2048Kb (2MB).
Email Size Limit (OnDemand): The email size limit for OnDemand licenses is 10MB. This limit has been set after considering the limited server resources since there are multiple helpdesks installed on one server sharing the resources.
Maximum number of emails to download in a batch: The setting defines the maximum number of emails that will be fetched upon single execution of the cron script. If you want to fetch large emails, value for 'Maximum number of emails to download in a batch' should be set to low. The default value is 20 i.e. 20 emails will be fetched for each email queue on single execution of cron script. The value should be kept low to prevent server load and memory dump.
The Maximum number of emails to download in a batch and Email Size Limit can be found at Admin CP > Settings > Email Parser - see image.
Note for Download license users: The value of 'Email Size Limit' depends upon the below mentioned PHP parameters:
These parameters are directly proportional to 'Email Size Limit' i.e. more the value of the it, more the resources required by PHP, hence the above mentioned settings need to be increased in accordance to the server resources.