Do you use the latest version of Lightroom 5. It's the version 5.7.1
My system say it's 5.7
I've fixed it. Somehow I'd changed the file format from jpeg to something else!
I've noticed this happening when the export format selected is "Original".
Just to explain: this option is not the same thing as selecting "JPG" when the image concerned reports "JPG", and so on.
It is a special provision whereby a simple duplicate is made of the imported file (of whatever type), with edits attached as metadata.
So there is no opportunity in that case, to control the pixel dimensions - nor anything else (except, how much metadata is to be attached)
Thanks for the advice which makes a lot of sense. I'm afraid I only know about 10% of the capability of Lightroom so I often come unstuck!