A simple mixin to generate font-size rules using rem and a px fallback. Read more here: http://snook.ca/archives/html_and_css/font-size-with-rem and here http://blog.typekit.com/2011/11/09/type-study-sizing-the-legible-letter/

_size.sass

=size($property , $size, $size-right: false, $size-bottom: false, $size-left: false)
  @if $size == $size-bottom and $size-right == $size-left //Check if we can discard size-bottom
    $size-bottom: false
  @if $size-right == $size-left //If horizontal sizes are equal we only need one of the properties
    $size-left: false
  @if $size and $size-right == false //All equal
    #{$property}: $size * 1px
    #{$property}: $size * 0.1rem
  @if $size and $size-right != false and $size-bottom == false // Vertical and Horizontal
    #{$property}: $size * 1px $size-right * 1px
    #{$property}: $size * 0.1rem $size-right * 0.1rem
  @if $size and $size-right != false and $size-bottom != false and $size-left == false //Different Verticals, Equal Horizontals
    #{$property}: $size * 1px $size-right * 1px $size-bottom * 1px
    #{$property}: $size * 0.1rem $size-right * 0.1rem $size-bottom * 0.1rem
  @if $size and $size-right != false and $size-bottom != false and $size-left != false //All four different
    #{$property}: $size * 1px $size-right * 1px $size-bottom * 1px $size-left * 1px
    #{$property}: $size * 0.1rem $size-right * 0.1rem $size-bottom * 0.1rem $size-left * 0.1rem

=font-size($size)
  +size('font-size' , $size)

=padding($size)
  +size('padding' , $size)

main.sass

html
  font-size: 62.5% //This way, 1rem = 10px, making it easier to define my text-sizes through the document

.some-rule
  +font-size(20) //20px or 2rem font-size
  +size('padding' , 20) //20px / 2rem padding
  +size('border' , 20, 10) //Vertical and Horizontal short borders
  +size('margin' , 30, 40, 50, 60) //All four different values for margin