Font-Awesome/scss/_sizes.scss

// icon sizes
// -------------------------

// scale icon size by n
@each $fa-icon-scale-name, $fa-icon-scale in $fa-var-scales {
  .#{$fa-css-prefix}-#{$fa-icon-scale-name} {
    font-size: #{$fa-icon-scale}em;
    line-height: #{$fa-icon-scale * 0.75}em;
    vertical-align: -15%;
  }
}

// multiply icon size by n
@each $fa-icon-multiplier-name, $fa-icon-multiplier in $fa-var-multipliers {
  .#{$fa-css-prefix}-#{$fa-icon-multiplier-name} {
    font-size: #{$fa-icon-multiplier}em;
  }
}