Here is a suggestion that I found in the DotNetNuke Forums ...
Change the maxRequestLength and requestLengthDiskThreshold to whatever you wish. The default timeout is in the machine settings but you can override it here. So here's an example, you could change it to this to allow 10 meg files with up to 20 minutes of time to upload:
< !-- allow large file uploads -- >
< httpRuntime useFullyQualifiedRedirectUrl="true" maxRequestLength="10240" requestLengthDiskThreshold="10240" executionTimeout="1200" / >