Purity III template defines some dimension layouts on site with the configuration from templates/purity_iii/less/variables.less file
// Extra small screen / phone
// Note: Deprecated @screen-xs and @screen-phone as of v3.0.1
@screen-xs: 480px;
@screen-xs-min: @screen-xs;
@screen-phone: @screen-xs-min;
// Small screen / tablet
// Note: Deprecated @screen-sm and @screen-tablet as of v3.0.1
@screen-sm: 768px;
@screen-sm-min: @screen-sm;
@screen-tablet: @screen-sm-min;
// Medium screen / desktop
// Note: Deprecated @screen-md and @screen-desktop as of v3.0.1
@screen-md: 992px;
@screen-md-min: @screen-md;
@screen-desktop: @screen-md-min;
// Large screen / wide desktop
// Note: Deprecated @screen-lg and @screen-lg-desktop as of v3.0.1
@screen-lg: 1200px;
@screen-lg-min: @screen-lg;
@screen-lg-desktop: @screen-lg-min;
// So media queries don't overlap when required, provide a maximum
@screen-xs-max: (@screen-sm-min - 1);
@screen-sm-max: (@screen-md-min - 1);
@screen-md-max: (@screen-lg-min - 1);
and you can easy to change the dimension layouts from that file.