By default, the maximum file size that can be uploaded as an attachment in Microsoft CRM 3.0 is 5Mb. To change this, I have found that there are really 4 steps:
1) Modify the following registry key (on the server) appropriately:
Key Path: HKEY_LOCAL_MACHINESOFTWAREMicrosoftMSCRMmaxuploadfilesize
Value: The value is the desired byte limit. 1Mb = 1048576 bytes so if you wanted the limit to be 20Mb the decimal value for the registry setting would be 20971520.
2) If you want the ability to upload more than 8Mb then you need to increase the .Net maximum request length by modifying the CRM web.config file. To do this, navigate to the folder containing the MS CRM web files on the server or use the IIS Manager to explore the folder contents (right-click on CRM website, select explore). Locate the web.config file and edit in Wordpad. Locate the “maxRequestLength” value and change it appropriately (this time in kb). For our example of 20Mb the value would be “20480”.
3) Change the “maxuploadfilesize” key on the client machine. Follow the instructions in step 1, but this time, the key is located here:
4) Restart the IIS services. Go to Start->Run->iisreset