How to keep grid cell height and width the same

I need to keep the Grid cell height = width when resizing.

The working code using a viewBox:

  <Viewbox>
        <Grid>
            <Grid.RowDefinitions>
                    <RowDefinition Height="1*"/>
                    <RowDefinition Height="1*"/>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto"/>
                    <ColumnDefinition Width="Auto"/>
                </Grid.ColumnDefinitions>
                <Label Grid.Row="0" Grid.Column="0" Background="Black" Width="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}"></Label>
                <Label Grid.Row="1" Grid.Column="0" Background="Gray" Width="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}"></Label>
                <Label Grid.Row="0" Grid.Column="1" Background="Gray" Width="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}"></Label>
                <Label Grid.Row="1" Grid.Column="1" Background="Black" Width="{Binding RelativeSource={RelativeSource Self}, Path=ActualHeight}"></Label>
            </Grid>
        </Viewbox>

Thank's to H.B. for the idea to use a viewBox! :)