You always want your main composition to be the same size as your biggest deliverable. Now days this means that for most work the smallest comp would be a standard HD 1920 X 1080 comp as the minimum size for most projects. Remember we're building videos here, not animated gifs or multi screen presentations. The exception to this rule would be if you were working on a project that originated as a 2K or 4K project. Here again, there are presets.
My standard process for animating a slide show like you describe is to first, resize the images using Lightroom/Bridge/Photoshop so that the images would be somewhere close to but not exceeding 100% scale, or, if you are going to be putting your images in 3D space, the images should not be closer to the camera than the Zoom value for the camera. This is the equivalent to 100% scale. If I had a bunch of 16 MPixel images from my DSLR that I wanted to use in a presentation and I wanted to see all of these images full frame in a standard HD comp, I'd scale them batch of them to about 2000 or 2500 pixels wide maximum. This gives me a little fudge factor to adjust position. If I had images that wanted to move in on so that I was cropping the image to about 1/4 of it's original frame then I would resize the images to about 4000 pixels wide. This means when the image is scaled to 100% in an HD comp we can only see about 1/4 of the image. As the images are resized I would use Photoshops image enhancement tools to restore detail and adjust the images so they would work well in a video. The exact settings depend on the amount of detail and contrast in the original image and it takes some practice to get everything perfect.
Once the images are resized I would bring them into AE and animate their position with some of my animation presets that use the in and out point of the layers and expressions to make nice moves on the images or I would simply keyframe position, rotation, scale in the case of 2D layers or camera or image position in the case of 3D layers. I would set in and out points for each image layer so they didn't hang around out of frame when they were not visible.
I hope this helps. There are lots of great plug-ins and scripts, and presets available to help you quickly create things like Ken Burns Style animations (I cringe every time someone uses that term because the technique of moving on still images was developed in the first year of moving pictures - about a hundred years ago)
Here's one of my favorite sets of animation presets from my good friend Stu to make Photo Montage easy and quick. Prolost Burns 1.6 for After Effects — Stu Maschwitz