Smaller, maybe, but with the caveat that doing so will reduce the quality of the images since you will be relying on the browser to downsize things via CSS (and browsers are lousy at re-sizing images). The image sizes for the slideshow are set in the theme PHP script files and the theme actually creates images from the ones you upload at that size.
Two slideshows side-by-side, no. CSS is a styling document used by browsers to position and style all the elements of a webpage. It cannot be used to add functionality, such as two slideshows side-by-side.
The few things I tried to resize the slideshow didn't work, so I would have to spend more time on it, and it might be that I can't resize it.