forcePlaceholderSize sortable option has no effect

I'm trying to apply a size to my placeholder class by css....
.placeholder {
width: 100px;
height: 100px;
}
but this has no effect, even when I have forcePlaceholderSize set to
true.
The size of the placeholder seems to be determined by the size of the
item being dragged.
Does anyone know how to size the placeholder??