Maybe this will help you consider the value of “responsive icon font icons”:
3 icons that are nice and big for large browser windows but don’t scale when the window is resized: http://screencast.com/t/bVF2qcroK (short video)
The solution would be the same whether the icon’s font-size is 300px or 2000px (or %, em, etc).
As far as I know, it’s not possible (via LESS/CSS) because if you go to any webpage and change the font-size to 2000px, all the letters in that paragraph will wrap, but they’ll also be wider than their container (e.g. a 2000px letter “W” is not going to stay within the 900px page width).