There are two ways to approach this.
You can influence the ligature:
<md-icon><h1>highlight</h1></md-icon>
This will resize the icon, but it will be affected by the styles set on the title elements (for example, font-weight). So, I would recommend resizing with CSS:
<md-icon style="font-size:24px">highlight<md-icon>
This will dramatically resize the icon. You can, of course, also add classes for this (which is even better!):
<md-icon class="large">highlight<md-icon>
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