Bootstrap v4. Responsive Sizing eg. w-md-50 w-lg-75

_sizing.scss

@each $breakpoint in map-keys($grid-breakpoints) {
  @include media-breakpoint-up($breakpoint) {
    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);

    @each $prop, $abbrev in (width: w, height: h) {
      @each $size, $length in $sizes {
        .#{$abbrev}#{$infix}-#{$size} { #{$prop}: $length !important; }
      }
    }
  }
}