Extension:Popups#Known problems says:
for a portrait image: exact (min) height 250 px & max width 203 px; for a landscape image: exact (min) width 320 px & max height 200 px[1]. To display, images must be able to become a thumbnail that is larger or equal to these "exact" sizes.
That is very convoluted and hard to interpret. Does "max" have anything to do with maximal? Maybe it uses language which corresponds closely to how the code is written but users have no need for such complications. I suspect the actual rule is simply something like this:
width × height must be either at least 320 × 200 px or 203 × 250 px.
If that is the rule then can the documentation please just say that?