public class MyRadCommandBar : RadCommandBar
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadCommandBar).FullName;
        }
    }

    public override Size GetPreferredSize(Size proposedSize)
    {
        if (this.Parent != null && this.Parent.AutoSize && (proposedSize.Width < 1 || proposedSize.Height < 1))
        {
            proposedSize = this.Size;
        }

        return base.GetPreferredSize(proposedSize);
    }
}