Theming Using Bootstrap Glyphicons and WordPress Dashicons

You can use em sizing to make the dashicons scale relative to the font size of their containing element. This should do the trick:

.dashicons:before {
    width: 1em;
    height: 1em;
    font-size: 1em;
}

Then if your <h1> element has a font-size of 48px, your dashicon will as well.