Stop pop-up from firing on smaller screens

I've just been trialling some pop-ups, and they look great on full-size screens.

However, they look bad on tablets and are completely un-closable on iPhones/phablets.

How can I stop "pop-ups" from firing it's code when the screen size is less than "x"?

(I'd also recommend this as tick-box or drop-in option for the next update.)