Hi, I'm glad my plugin has you excited!
You don't have to set the dimensions in each slideshow, that is only if each slideshow is going to be a different size. If all your slideshows will have the same dimensions, you only need to set them on the options page.
The plugin is optimized for uniformly sized slides, but you don't have to do this, they can be any size you want. The only catch is that any slides larger than the slideshow dimensions will be cropped to fit. So as long as it is sized to fit your largest slide, you'll be OK.
This works best if the slide sizes are pretty close, like a collection of photos with a mix of portrait and landscape images. Any slides that are smaller than the slideshow will be centered horizontally and aligned to the top vertically.
If you want to have a border around the slideshow, but have it fit different sized images, I would suggest adding the border to the images themselves rather than the slides or slideshow.