Thanks to viper for the advice to,

use the Crop tool set it to he high/width

Yes, that would help to get a final image that matches to paper size (or some arbitrary image size requirement). 

OTOH, I wanted to be able to select (crop) the image without knowing the image size in advance.  ... and still get a specific aspect ratio.  Up until now, My results depended on guess or aproximation of the aspect ratio when using the crop tool freehand (and estimating the height and width from the rulers) ... the advice I got in this thread will make my image selection(s) more effective ... more precise ... even easier ...

--
Martin