To properly resize your Keynote-created e-poster and export it as a PDF file ready for submission, please follow the steps shown below.
1. In Keynote click on the Inspector button to bring up the Inspector palette.
2. Choose "Custom slide size..."
3. You want to set the dimensions of your e-poster so that they fit optimally on the monitors where your e-poster will be shown. Set the width to 1920 pixels and the height to 1000 pixels.
4. After you finish your e-poster, go to File » Export » PDF...
5. Important: On the window that pops up, deactivate the option "Add borders around slides" or Keynote will add space around your slide. Furthermore, for optimal results, it's good practice to choose "Best" for the Image Quality.
6. Choose a filename for your e-poster, and the folder where you want it saved (e.g.: your Documents folder or your Desktop). Your file is now ready for submission.
If, when submitting this file, you are shown a message stating that the dimensions of your e-poster are different from the ones specified by the Conference, it is safe to ignore it, as long as you have followed closely the steps above.