responsive-font.css

body {
  font-size: 5vw;
}
@media (max-width: 700px) {
  body {
    font-size: 35px;
  }
}
@media (min-width: 3000px) {
  body {
    font-size: 150px;
  }
}

responsive-font.sass

///
/// Viewport sized typography with minimum and maximum values
///
/// @author Eduardo Boucas (@eduardoboucas)
///
/// @param {Number}   $responsive  - Viewport-based size
/// @param {Number}   $min         - Minimum font size (px)
/// @param {Number}   $max         - Maximum font size (px)
///                                  (optional)
/// @param {Number}   $fallback    - Fallback for viewport-
///                                  based units (optional)
///
/// @example scss - 5vw font size (with 50px fallback), 
///                 minumum of 35px and maximum of 150px
///  @include responsive-font(5vw, 35px, 150px, 50px);
///
@mixin responsive-font($responsive, $min, $max: false, $fallback: false)
  $responsive-unitless: $responsive / ($responsive - $responsive + 1)
  $dimension: if(unit($responsive) == 'vh', 'height', 'width')
  $min-breakpoint: $min / $responsive-unitless * 100
  
  @media (max-#{$dimension}: #{$min-breakpoint})
    font-size: $min
  
  @if $max
    $max-breakpoint: $max / $responsive-unitless * 100
    
    @media (min-#{$dimension}: #{$max-breakpoint})
      font-size: $max
  
  @if $fallback
    font-size: $fallback
  
  font-size: $responsive

// -------------------- DEMO --------------------------
body
  @include responsive-font(5vw, 35px, 150px)