SeeDMako: Of course :// It stil has 387 kB.
I downloaded The Witcher 2 trough the downloader just now to check out the chunk sizes and the file in question at my end is exactly the same size as yours, 387 KB, as you can see in the pic. The downloader merged the chunks into install files without any error so it would seem the problem lies elsewhere.
According to the error message you described in your first post the problem seems to be with the merging of the first bin file so other than deleting and downloading all those chunks again, all the 3.0.1.17-1 files, i'm not sure what you could do.
There's a topic about issues with the downloader in the general board, maybe you should post there too.
Edit: Wait, the merging progress bar went all the way up but then came down when i wasn't paying attention and in the end it did reported verification errors or something like that. The files seem to have merged correctly though, and their sizes match those of the files i downloaded through the browser, if it's the same at your end maybe try running the setup anyway ? You can do that by going into the gog downloader folder and double clicking the setup.exe file