Change video size in sequence settings ??

I want to animate some still images and want the video size to be 1080 (W) x 1920 (H).  I've imported an image that is roughly that size but now I want to change the video size.  When I go to Sequence Settings to do that, the aspect ratio of the image is maintained and one of the dimensions adjusted accordingly.  I don't want that.  How can I force the video to be 1080x1920?  Thanks.