1TB is definitely too big...it's going to create (or try too based on available space) a 1TB file which is going to take a very long time to create.
You can set it to 4GB, or even 10GB which I have done before. It's a nice middle-ground size for the files that are created. It's also going to mean moving them around is a lot easier too.
Setting it lower means BEWS will break it down into 10GB files...create a 10GB file, write to it, create another 10GB file, write to it until it's completed the backup.
Hope this helps?