Recently I had to do some work on a legacy application for a client and they were getting errors when trying to upload some files that I would consider to be of relatively small sizes of around 300kb. It turns out that IIS 6 is configured by default to a maximum upload size of 204,800 bytes (200kb) which is of little use these days. The fix I implemented required me to open the metabase.xml file located in c:WindowsSystem32Inetsrv and locate the setting AspMaxRequestEntityAllowed and amend the value to something more suitable. For good measure I then did an IIS reset and the problem went away.
I later discovered a Microsoft Knowledgebase article (327659) with further details on this and an alternative method of making the same change described above that you may wish to read before implementing any changes yourself.