From the Blackbaud NetCommunity Website design guide:

Warning: To prevent possible Denial of Service attacks, there is a limit to the size of the files you can upload. The default is typically set to 4096 KB, but the system administrator can adjust it up to 10,000 KB. We do not recommend you go over this size.
The administrator can specify this limit in the Web.Config file.

For example, to increase the file size, add maxRequestLength="8192" (where 8192 is the file size) to the <httpRuntime useFullyQualifiedRedirectUrl="true" /> key.

For very large files, increase the website timeout setting. For example, to increase the timeout setting, add
executionTimeout="120" (where 120 is the duration in seconds) to the <httpRuntime useFullyQualifiedRedirectUrl="true" /> key.

Before you maximize the document file sizes or website timeout settings, visit and review the Microsoft Knowledgebase article.