How to autosize the height of a list view in XAML

Set only ListView property HasUnevenRows="True"

You need to add HasUnevenRows to True and let unset the RowHeight property.

<ListView ItemsSource="{Binding List}" VerticalOptions="FillAndExpand" HasUnevenRows="True" SelectedItem="SelectedCTR" SeparatorVisibility="None">
    <ListView.ItemTemplate>
        <DataTemplate>
            <ViewCell>
                <Grid>
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto"/>
                        <RowDefinition Height="Auto"/>
                    </Grid.RowDefinitions>
                    <Image Grid.Row="0" HeightRequest="100" MinimumWidthRequest="160" WidthRequest="160" Source="{Binding AttachedmentData,Converter={StaticResource stringToImage}}" />
                    <StackLayout Grid.Row="1" VerticalOptions="FillAndExpand">
                        <Label Text="{Binding Number}" Font="19"
                             TextColor="#f35e20" />
                        <Label Text="{Binding TrimmedSynopsis}" Font="17"
                             TextColor="#503026" />
                    </StackLayout>
                </Grid>
            </ViewCell>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>