Linux and Windows VDS/Dedicated Servers

Login to the Plesk Control Panel
Click "Server" on the left hand side
Click "Mail"
Under the "Preferences" Tab you will see the section "General Options"
Increase the value next to "Maximum Letter Size"


The field accepts Kilobytes
Kilobytes
Megabytes
1024
1
5120
5
8192
8
10240
10
15360
15
20580
20
51200
50