Binding left of margin

You can use a converter:

<Page.Resources>
    <local:MarginConverter x:Key="MarginConverter" />
</Page.Resources>

[...]

Margin="{x:Bind classsize, Converter={StaticResource MarginConverter}}"

converter implementation, replace YourClass with your actual class

public class MarginConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    {
        var classsize = (YourClass)value;
        return new Thickness(classsize.widtharea, 20, 0, 0);
    }

    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        throw new NotImplementedException();
    }
}