Make your images as large as possible to have maximum impact in Slide Show mode.
The slide show window has a minimum format of 960×360 pixels. But you can insert images up to 1920×1080.
When you look at the slide show in standard view, any images larger than 960×360 will be cropped. But when you click/tap on the slide show in Preview or at the Conference, it will open up as an overlay, and then it will show all images in their full size.