adaptive-font-size.scss

:root {
 font-size: calc(0.6em + 1vw)
}

@mixin for-size($range) {
  $phone-upper-boundary: 600px;
  $tablet-portrait-upper-boundary: 900px;
  $tablet-landscape-upper-boundary: 1200px;
  $desktop-upper-boundary: 1800px;

  @if $range == phone-only {
    @media (max-width: #{$phone-upper-boundary - 1}) { @content; }
  } @else if $range == tablet-portrait-up {
    @media (min-width: $phone-upper-boundary) { @content; }
  } @else if $range == tablet-landscape-up {
    @media (min-width: $tablet-portrait-upper-boundary) { @content; }
  } @else if $range == desktop-up {
    @media (min-width: $tablet-landscape-upper-boundary) { @content; }
  } @else if $range == big-desktop-up {
    @media (min-width: $desktop-upper-boundary) { @content; }
  }
}

// usage
.my-box {
  padding: 10px;
  
  @include for-size(desktop-up) {
    padding: 20px;
  }
}

@mixin for-phone-only {
  @media (max-width: 599px) { @content; }
}
@mixin for-tablet-portrait-up {
  @media (min-width: 600px) { @content; }
}
@mixin for-tablet-landscape-up {
  @media (min-width: 900px) { @content; }
}
@mixin for-desktop-up {
  @media (min-width: 1200px) { @content; }
}
@mixin for-big-desktop-up {
  @media (min-width: 1800px) { @content; }
}