What size should by Slides be?

You want an image that's 1100 pixels wide by 300 pixels tall.

Be sure to use the "Save for Web" feature of your image editor. This will produce a much smaller file size (faster load time) without visually affecting the quality of the image.

If you're having a hard time "making it work" for that size, consider two options:

  1. Resize the image, but make sure you UN-select "constrain proportions," or whatever it's called in your graphic editing program (it's called "Scale proportionally" in Pixelmator). It basically allows you to change the height to 300 without also changing the width in the process. That gives you the squished version.
  2. Use the crop tool to select a sub-section of the image that's full width (1100) and 300 high.