Here are some simple steps to enlarge the max attachment size in Microsoft Dynamics CRM.
1. Open the web.config of MS CRM
2. Find the line <httpRuntime executionTimeout="300" maxRequestLength="8192" />
3. Change the value of “8192” to the desired max size in kilobytes. For this example it is set to “20480”.
4. Open CRM, go to System settings. You can now change the max size of attachments in the E-mail tab.