Webkit-mask-size not working

Webkit-mask-size not working

I'm trying to use webkit-mask-size to make a mask image smaller. Like this:
.myClass {
width: 100%;
height: 100%;
background: #ffffff;
-webkit-mask-size: 50% 50%;
-webkit-mask: url(../css/images/myimage.png) center center;
}
The div which has myClass applied to it has a parent container which has a
fixed height set on it.
Whatever I set -webkit-mask-size to it makes no difference.