In the export dialog it lets me set the dpi and WxH. The exported file is as specified, but the sheet is just a small part of the image.
I was able to do this the other day, but now I can’t remember what steps I used to get the sheet to fill the image size 
Dennis
you should checkbox “Selection” in dialogue Export.
Select range of cells, select menu File->Export, select type of file JPEG, active checkbox “Select”, type name of new file and push button Export
That doesn’t work either. I’m setting the dpi to 300. The sheet is set up to fit 11x8.5 paper. On export the sheet fills less than a quarter of the page. Of course print preview and printing at 96 dpi fills the page and looks reasonably well, but this is destined for a print service that will want 300dpi.