According to the section "Slideshow" of the uOttawa Web Style Guide, the images need to be "scaled consistently with the other images in the same slideshow (normal aspect ratio is 16:9, with a crop size 1523px x 860px; panoramic aspect ratio is 9:3, with a crop size of 1523px x 506px)".
That said, as some may have discovered, the aspect ratio 16:9 will result in slideshow images that have a rather cumbersome height. So most faculties and services have set their own specifications:
Essentially, the key is to make sure that the images within a same slideshow are consistent in size. If you are not sure what size of image your service or faculty uses, please communicate with your webmaster or contact person.
For information: For a page with an M layout (body = 75%; sidebar = 25%), the maximum width that the image can take up is 731 px. For a page with a B layout, (body = 100%), the maximum width is 967 px.