How to animate size parameter of ffmpeg zoompan filter? How is it possible to dynamically evaluate the s (output image size) parameter of ffmpeg zoompan filter on each frame?

The parameter expectes its value in "WxH" format i.e. "1920x1080". 
How to substitute an expression that would update the size as a function of, say, frame number?

Couldn't find a way to concatenate strings.

For a single number parameter I can do smth like y='(out_h-in_h)/2', but how to take two expressions and join them with an "x" string?
The zoompan size parameter cannot be animated. It has to be a fixed numeral size of the form WxH, or a size label like hd1080.