Y-only maximum dimension

Is it possible to specify that the .sized maximum image dimension will apply only to a single axis? I have an album of panoramas, and I'd like the sized intermediate images to be XXXX x 640, so they fill the screen vertically. I.e. whatever the aspect ratio of the picture, it is always 640 pixels high, and however wide is necessary. Right now, specifying a maximum dimension of intermediate image, scales them all to the same width (since that is the longer dimension).