max-width using em

Em is just another unit. It depends on the font size. Current font size (e.g. 14px) = 1em. 2em doubles the font size (28px).

Have a look at your screen resolution: 1920/140 = 13,714285714 - looks like your current font size is 14px. Therefore setting max-width:140em is the same like max-width:1920px or max-width:100% - in your special case.

This site, http://pxtoem.com, makes conversion and has a table of each size.