How to decode image with widget size in AspectRatio widget?

Question

I am using AspectRatio widget to size images in my app. Here's the code:

Widget _buildThumbnail() {
    return AspectRatio(
        aspectRatio: 1,
        child: CachedNetworkImage(
            imageUrl: artist.thumbnail,
            fit: BoxFit.cover,
        ),
    );
}

Images are large, and I want to use image-widget size as image decode size. I do not know what the image-widget size is before hand. This widget is loaded in a GridView with crossAxisCount of 2, 3, and sometimes 4.

Is there a way to tell cached_network_image or Image widget to auto decode image according to widget size? Or a way to get size of AspectRatio's child?