har-
By default DNN caps upload/downlaod file sizes at 8MB.  You can allow larger file sizes by increasing this limit in your web.config file.
 
Open your web.config and find the following entry:
 
 <httpRuntime useFullyQualifiedRedirectUrl="true" maxRequestLength="8192" requestLengthDiskThreshold="8192"/>

   
Change maxReqestLangth and requestLengthDiskThreaeshold from "8192" (8 megabytes) to "81920" (80 megabytes).  Also add an executionTimeout entry to increase the default timeout value.  This value is measured in seconds so for large file size support you can try a value of "1200" for 20 minutes.  You'll end up with the following:

 <httpRuntime useFullyQualifiedRedirectUrl="true" maxRequestLength="81920" requestLengthDiskThreshold="81920" executionTimeout="1200"/>
 
 
Once you changed this and saved the web.config file DNN should automatically recycle itself and immediately start supporting the larger file sizes.

Cheers!
-mamlin

you should use a different upload module, using either Silverlight, Flash or AJAX upload controls.