Callout extends Container, so you shouldn't have problems setting the width and height to 100%.
Also, in the Callout code there's a method:
/**
* @private
* Force a new measurement when callout should use it's screen-constrained
* max size.
*/
private function invalidateMaxSize():void
{
// calloutMaxWidth and calloutMaxHeight don't invalidate
// explicitMaxWidth or explicitMaxHeight. If callout's max size changes
// and explicit max sizes aren't set, then invalidate size here so that
// callout's max size is applied.
if (!canSkipMeasurement() && !isMaxSizeSet)
skin.invalidateSize();
}
So, try to set the 'explicitMaxWidth' and 'explicitMaxHeight' properties.