I have no idea about your campaign problem, but I do remember such similar issues have been posted before on THW. Just search around and you'll find them.
About the campaign limit: I can guarantee you that you have unlimited file size for campaigns. Battle.net and LAN games are restricted to about 4MB, but campaigns are Single Player, so that doesn't affect them.
As for the uploading file size on THW: There's a maximum of 5MB for maps and 10 or 15MB for campaigns. For any campaign with a lot of imports, you'll easily exceed this limit, but don't worry. There are a lot of ways to do it. Extremer file sizes call for extremer measures.
10-15MB campaigns should just be uploaded as attachment of map.
Above that limit, you can use external file uploading sites like MediaFire, FileFactory or RapidShare. These have a very large upload limit and all you have to do is paste the link to the file it gives you, where you want to have the link placed! These do however have upload limits, because they're not infinite either.
The last drastic measure (in case external file uploading sites don't do the job due to speed and/or file size) is to make a torrent of it and upload the torrent file (only a few kB). This does mean that people can only download while there are seeders (people who constantly upload pieces of the map for downloaders to download). At first, you'll be the only seeder, but once people finished downloading, they automatically seed along while their torrent program is running and the file isn't stopped by them.
Torrents aren't known by everyone and you need to download a certain torrent program (like uTorrent or BitTorrent) do download .torrent files and thus download the file, so I suggest you avoid using this technique.