I see that Tanchelyn advised,

Do this with the rectangular selection tool.

Thanks!  That is very good to know.  I tried it and see how it works. 

"click -> drag -> release" sets the selection size

"click -> drag -> release" then moves the selection as needed. 

Without starting over, is it possible to adjust the selection size (to a larger or smaller area with the same preselected aspect ratio) after the initial "click -> drag -> release"?

--
Martin