With Slickr Flickr Standard Edition you can control the size of a slideshow by placing it in a ‘DIV’ element that constrains the slideshow dimensions. In the Pro Edition you can get the same affect more easily using the ‘height’ and ‘width’ parameters.
This feature is particularly useful when you have odd sized photos (not 4×3 or 3×4 format).
Here is a slideshow of square photos of Roma. I have chosen the Medium 640 size from Flickr and am scaling them down slightly to 600px square
slickr-flickr type="slideshow" search="sets" set="72157625412406257" items="10" size="m640" height="600" width="600" captions="off" border="on" bottom="40"