...or do you need create your own classes in a separate .css file?
Do you mean like font awesome has the fa-lg, fa-2x, fa-3x, fa-4x, and fa-5x?
I've been thinking of adding them, seems like it may be useful.
Yes.
I added a custom css based on
http://google.github.io/material-design-icons/
.mdi-18px { font-size: 18px; }
.mdi-24px { font-size: 24px; }
.mdi-36px { font-size: 36px; }
.mdi-48px { font-size: 48px; }
.mdi-dark { color: rgba(0, 0, 0, 0.54); }
.mdi-dark.mdi-inactive { color: rgba(0, 0, 0, 0.26); }
.mdi-light { color: rgba(255, 255, 255, 1); }
.mdi-light.mdi-inactive { color: rgba(255, 255, 255, 0.3); }
works pretty well with angular material.
(the default icon size was a bit too small)
<md-button class="md-fab md-mini mdi mdi-pencil-box-outline mdi-24px"
ng-click="show()">
</md-button>

I'll get it added to the next release, which should be this week.
Closing this issue. It is in the build script now and will go in the latest release later this week.
Most helpful comment
Yes.
I added a custom css based on
http://google.github.io/material-design-icons/
works pretty well with angular material.
(the default icon size was a bit too small)