The following numeric meta fields can be used with comparisons <, <=, >, >=, = (equals), != (not equals):
filesize: The number of bytes, if known in advance
filesize_approx: An estimate for the number of bytes
width: Width of the video, if known
height: Height of the video, if known
aspect_ratio: Aspect ratio of the video, if known
So a height<=1080 should be it.