The Slimbox2 code that’s used by WP-Slimbox2 is designed to display the image pointed to by the URL, in this case you’re pointing to the 1600×1600 images, so that’s what it displays. If you’d like to display images at a smaller size, I’d recommend you also upload images of the size you’d like to display, point to those with your URLs, and if you still want to include links to the larger image, do so in the caption.
I’ve been making a small modification to the Slimbox2 code myself that will implement a resize option that fits to the screen, but I don’t know when I’ll have a chance to release it.