@import url('https://fonts.google.com/specimen/Source+Sans+Pro?selection.family=Source+Sans+Pro');

$bj-default-font-family: 'Source Sans Pro', sans-serif;
$bj-default-font-color: #363535;
$bj-dark-font-color: #34383D;
$bj-invert-font-color: #FEFEFE;

$bj-normal-font: 20px;
$bj-big-font: 26px;

.bj-normal-font {
    font-size: $bj-normal-font;
}

.bj-big-font {
    font-size: $bj-big-font;
}


@import url('https://fonts.google.com/specimen/Source+Sans+Pro?selection.family=Source+Sans+Pro');

$bj-default-font-family: 'Source Sans Pro', sans-serif;
$bj-default-font-color: #363535;
$bj-dark-font-color: #34383D;
$bj-invert-font-color: #FEFEFE;

$bj-normal-font: 1em;
$bj-big-font: 1.4em;

.bj-normal-font {
    font-size: $bj-normal-font;
}

.bj-big-font {
    font-size: $bj-big-font;
}


Use em for font size.
Use em for font size.