Hi John,
I think i may have unwittingly stumbled upon the answer to the question i raised yesterday.
It looks to me that the standard preview URL's were automatically increased in size by 25% between May/June of this year as the 2 examples above suggest. Perhaps it is not possible to vary the size of these pictures.
My preview pics will automatically be smaller when i am doing archive work.
In any event, don't waste any more time on it--i know you are busy working on the latest preview Blogs.
Cheers!
Peter