Am I right in thinking that as long as I set the width to be smaller than that in the Configuration Files/Images Settings then this will work.
I did not want to resize the width of thumbnails across the whole website.
The other thing is: I have deliberately not set the thumbnail height.
As a newbie HikaShop user I was not sure about this but both height and width require resizing in the case of portrait view thumbnails. And I don't think it is possible to be so specific (though understandable - things are complex enough).
Ken