Exporting problem.

Previously when uploading images to Shutterstock or Wirestock I often had to change the size because the image was too big or small to upload. I would export the new revised file with no problem. Now after a recent Lightroom update I am unable to export images and change the size. Small jpeg, large jpegs and tiffs all export at the same size when exporting from Lightroom to my desktop on my iMac. 
Any suggestions on how to resolve this problem would be greatly appreciated. Thank you.