First, VB is the wrong language to use for the project.

That said, how large will the largest file be? VB can only handle filesizes up to 2GB.

Also, why split the files up? The reason download managers do this is because they're downloading multiple pieces of the file at the same time, not just to break the file up. Do you know how to do a resumed download in FTP? (That's how you download more than 1 piece of the file at a time.)