@fortawesome/fontawesome-free/less/_sizing.less

// sizing icons
// -------------------------

// literal magnification scale
.sizes-literal(@factor) when (@factor > 0) {
  .sizes-literal((@factor - 1));

  .@{fa-css-prefix}-@{factor}x {
    font-size: (@factor * 1em);
  }
}
.sizes-literal(10);

// step-based scale (with alignment)
each(.fa-sizes(), {
  .@{fa-css-prefix}-@{key} {
    .fa-size(@value);
  }
});