By reading the material design in github I found this useful stuff that might help you.
/* Rules for sizing the icon. */
.material-icons.md-18 { font-size: 18px; }
.material-icons.md-24 { font-size: 24px; }
.material-icons.md-36 { font-size: 36px; }
.material-icons.md-48 { font-size: 48px; }
/* Rules for using icons as black on a light background. */
.material-icons.md-dark { color: rgba(0, 0, 0, 0.54); }
.material-icons.md-dark.md-inactive { color: rgba(0, 0, 0, 0.26); }
/* Rules for using icons as white on a dark background. */
.material-icons.md-light { color: rgba(255, 255, 255, 1); }
.material-icons.md-light.md-inactive { color: rgba(255, 255, 255, 0.3); }
From the code above, there you can simply change or override the material css icons.
Sample code:
<i class="material-icons md-18">face</i>
More details here
Change the icon size of Material Design icon