The message size limits on a send connector can be set through the Exchange Management Console via Organization Configuration > Hub Transport > Send Connectors by opening the Properties of the Send Connector:
You can also use Get-SendConnector to view the setting:
The default maximum sending message size for Exchange 2007/2010 send connectors is 10 MB. If you want to be able to send larger messages over this send connector, you can use the EMC or Set-SendConnector:
Set-SendConnector –Identity Internet –MaxMessageSize 25MB
Valid range for this setting is anywhere in the range between 64KB and 2GB or Unlimited.
The message size limits on the organization level can be set through the Exchange Management Console via Organization Configuration > Hub Transport > Global Settings by opening the Properties of Transport Settings:
Of course, you can also view the settings using the Exchange Management Shell using Get-TransportConfig
As you can see, the default value in Exchange 2010 is 10240 (10 MB) for both receive as well as send message sizes. If you require a higher value, for example to enable people to send and receive larger attachments, you can use the EMC or Set-TransportConfig: