Same place you setup email in the beginning
CWIS > Properties > Connectivity > Protocols > SMTP Server > Maximum data size per email
10 MB is the default, 20 MB is the max (sadly). That said, your email settings are way to high iof 5 pages hit 10 MB. I highly suggest a lower DPI and/or higher compression, you don't need 400-600DPI to read a pdf, and you shouldn't use email to make high quality scans for reprint later, that is what scan to SMB/FTP/USB are there for.
This machine will have a larger scan size (always) than a Xerox Workcentre product, the Color products use less compression even when set to their Max to achieve a higher quality scan.