This is a bit thorny.  I think I'd be tempted to write something like:
```js
var ptPerPx = 72 / 96;
var targetHeight = 2 * style.pointRadius * ptPerPx;
if (fill.src !== style.externalGraphic ||
    !fill.size || parseFloat(fill.size.split(",")[0]) !== targetHeight) {
    // ...
```