PrimeFaces components internally use PrimeIcons library, the official icons suite from PrimeTek.
PrimeIcons use the pi pi-{icon} syntax such as pi pi-check. A standalone icon can be displayed using an element like i or span
<i class="pi pi-check"></i>
<i class="pi pi-times"></i>
Size of the icons can easily be changed using font-size property.
<i class="pi pi-check"></i>
<i class="pi pi-check" style="font-size: 2rem"></i>
Special pi-spin class applies continuous rotation to an icon.
<i class="pi pi-spin pi-spinner" style="font-size: 2rem"></i>
Here is the current list of PrimeIcons, more icons are added periodically. You may also request new icons at the issue tracker.