How do I size images for Adobe Premiere Elements 12

Photoshop includes Film&Video preset templates for many formats, though not necessarily all inclusive or up to date. Not sure if presets are included in Elements version? As mentioned, set your Photoshop(PS) workspace to match your video project. Generally, you can easily modify one of PS presets for your specific needs if necessary.

I'd also recommend saving any photos or graphics from Photoshop in PNG, a lossless format, if used in your video project.