When I do slideshows I never resize anything of any format. I set the project by sliding image files onto the timeline. It works. At output I match the screen I want to view on. Premiere Elements is very adaptable to unsized still images.
Letterboxing can be a function of the wrong choice in the checkbox "Default Scale to Frame Size" in Edit > Preferences > General.
When Grisetti posts here he usually says to resize to something a little larger than the intended project. 1000 x 700 is too small for anything we now use to watch slideshows on. Again, I see no reason to resize, but if you do make it larger than the project you want and maintain the aspect ratio. For current HD TVs, anything smaller than 1920x1080 is too small.