I've managed to find the Impulse error log entries for this, you can find them below. My goal was to install the latest beta version of Elemental, so I had enabled beta versions in the Impulse preference panel. Now I have disabled that setting, restarted Impulse and I'm downloading the stable version instead. I hope that I can then install the beta via an auto-update instead of a full install. I'll post an update when my download succeeds or fails.
The thing in the logs is that all the errors are the same except for the time at which they occur. Even the amount of bytes downloaded is the same. Could it be that the expected file size is based on either the full retail version of Elemental or an older beta?
-- [23-11-2010 21:56:24] ----------------------------------------------------
Context: DownloadFile - File Download (Web)
Message: File downloaded is smaller than expected
C:\ProgramData\Stardock\Impulse\Temporary\DownloadQueue\elemental\elemental1.09n[b].108.impulse
File size expected: 2028404904 File size received: 1156841472
Build #: 1976
-- [23-11-2010 21:56:24] ----------------------------------------------------
Context: Elemental: War of Magic
Message: File downloaded is smaller than expected
Build #: 1976
-- [23-11-2010 22:35:29] ----------------------------------------------------
Context: DownloadFile - File Download (Web)
Message: File downloaded is smaller than expected
C:\ProgramData\Stardock\Impulse\Temporary\DownloadQueue\elemental\elemental1.09n[b].108.impulse
File size expected: 2028404904 File size received: 1156841472
Build #: 1976
-- [23-11-2010 22:35:29] ----------------------------------------------------
Context: Elemental: War of Magic
Message: File downloaded is smaller than expected
Build #: 1976
-- [23-11-2010 22:51:05] ----------------------------------------------------
Context: DownloadFile - File Download (Web)
Message: File downloaded is smaller than expected
C:\ProgramData\Stardock\Impulse\Temporary\DownloadQueue\elemental\elemental1.09n[b].108.impulse
File size expected: 2028404904 File size received: 1156841472
Build #: 1976
-- [23-11-2010 22:51:06] ----------------------------------------------------
Context: Elemental: War of Magic
Message: File downloaded is smaller than expected
Build #: 1976