From what I can see in the documentation, there doesn't appear to be support for multiple thumbnail sizes. Personally, I wouldn't rely on BackLoad for creating your thumbnails, anyways. It's really more of a corollary feature for BackLoad, rather than something they really focus on, anyways.
Instead, use something for this that is designed for the purpose, like ImageResizer. That will let you create thumbs on the fly in whatever sizes you like, anytime you like -- much more flexible.