Use viewport size in a setvar

[ZiNgA BuRgA]

You can't.  Template conditionals are completely server side, Javascript is client side.

You can write template variables into Javascript eg

Code:

var imgUrl = <?=json_encode($GLOBALS['threadfields']['coverpic']['value'])?>;

---

but not the reverse (without making additional requests).

You'd probably have to write the code in Javascript.