Please find below the information that is available in the help topic under 'System Limits and maximums'. I have taken from PRPC 7.2 help. In earlier versions it is limited to 25 megabytes only.
You cannot attach a file larger than 1 gigabyte to a work item. Similarly, this limit applies when using the Upload button on Rule-File-Text, Rule-File-Binary, and Rule-File-eForm rule forms, and with the Upload button on the Import landing page tab. You can enforce a higher or lower limit on your system by changing the value for the Initialization/MaximumFileUploadSizeMB parameter in the prconfig.xml file or Dynamic System Setting.
I am not sure whether the below link will be helpful for you or not as it does not specifically speaks about email attachments. However, it implies the entire email size as can be listed under Services/EmailMaxSize of the process tab in an email listener.