How do you select the right size icon from a multi-resolution .ico file in WPF?

If the reason you're asking is that the icon looks blurry to you, check out this very good article on the topic that I used to solve that problem: http://blogs.msdn.com/dwayneneed/archive/2007/10/05/blurry-bitmaps.aspx

You will have to use a custom control that not only sizes the icon exactly, but ensures that it coincides exactly with the pixel grid. Only then will you avoid interpolation and therefore blurriness.

Trying to find some info on your query about image size selection in icons...will post back if I find any...

It doesn't appear to be possible using Xaml only.

I use simple Markup Extension for that:

/// <summary>
/// Simple extension for icon, to let you choose icon with specific size.
/// Usage sample:
/// Image Stretch="None" Source="{common:Icon /Controls;component/icons/custom.ico, 16}"
/// Or:
/// Image Source="{common:Icon Source={Binding IconResource}, Size=16}"
/// </summary> 
public class IconExtension : MarkupExtension
{
    private string _source;

    public string Source
    {
        get
        {
            return _source;
        }
        set
        {
            // Have to make full pack URI from short form, so System.Uri recognizes it.
           _source = "pack://application:,,," + value;
        }
    }

    public int Size { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        var decoder = BitmapDecoder.Create(new Uri(Source), 
                                           BitmapCreateOptions.DelayCreation,
                                           BitmapCacheOption.OnDemand);

        var result = decoder.Frames.SingleOrDefault(f => f.Width == Size);
        if (result == default(BitmapFrame))
        {
            result = decoder.Frames.OrderBy(f => f.Width).First();
        }

        return result;
    }

    public IconExtension(string source, int size)
    {
        Source = source;
        Size = size;
    }

    public IconExtension() { }
}

Xaml usage:

<Image Stretch="None"
       Source="{common:Icon Source={Binding IconResource},Size=16}"/>

or

<Image Stretch="None"
       Source="{common:Icon /ControlsTester;component/icons/custom-reports.ico, 16}" />

(based on @Nikolay great answer and follow-up comment about binding)

You probably will be better off creating a Converter instead of a MarkupExtension so that you can leverage Binding. Using the same logic as provided by @Nikolay

    /// <summary>
    /// Forces the selection of a given size from the ICO file/resource. 
    /// If the exact size does not exists, selects the closest smaller if possible otherwise closest higher resolution.
    /// If no parameter is given, the smallest frame available will be selected
    /// </summary>
    public class IcoFileSizeSelectorConverter : IValueConverter
    {
        public virtual object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            var size = string.IsNullOrWhiteSpace(parameter?.ToString()) ? 0 : System.Convert.ToInt32(parameter);

            var uri = value?.ToString()?.Trim();
            if (string.IsNullOrWhiteSpace(uri))
                return null;

            if (!uri.StartsWith("pack:"))
                uri = $"pack://application:,,,{uri}";

            var decoder = BitmapDecoder.Create(new Uri(uri),
                                              BitmapCreateOptions.DelayCreation,
                                              BitmapCacheOption.OnDemand);

            var result = decoder.Frames.Where(f => f.Width <= size).OrderByDescending(f => f.Width).FirstOrDefault()
                ?? decoder.Frames.OrderBy(f => f.Width).FirstOrDefault();

            return result;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }

You must then create a resource from your converter class somewhere in a ResourceDictionary as usual:

<localConverters:IcoFileSizeSelectorConverter x:Key="IcoFileSizeSelector" />

And then you can use Binding:

<Image Source="{Binding Path=IconResource, Converter={StaticResource IcoFileSizeSelector}, ConverterParameter=16}" />

PS: in the converter code, we accept all inputs for parameter, even missing or invalid ones. That behaviour is more convenient if like me you like to play with live XAML edit.