Fullscreen placeholder

Hello,
First of all, thanks for creating this awesome library!
I am using CachedNetworkImage for viewing images on a pageview

```
CachedNetworkImage(
imageUrl: image.url,
placeholder: (context, url) => new SizedBox(
width: 64.0,
height: 64.0,
child: new LinearProgressIndicator(),
),
errorWidget: (context, url, error) => new Icon(Icons.error),
),

```
The problem is even though I've specified width and height of placeholder container, it seems to be ignoring those parameters. My circular progress indicator covers entire screen of application. Is this a bug or am I doing something wrong?

Thanks.