To reproduce:
- Add some controls to a command bar.
- Get the PreferredSize like this:

 Console.WriteLine(radCommandBar1.PreferredSize.ToString());

Workaround:
class MyCommandBar : RadCommandBar
{
    public override Size GetPreferredSize(Size proposedSize)
    {
        if (proposedSize.Width == 0 && proposedSize.Height == 0)
        {
            return this.Size;
        }
        return base.GetPreferredSize(proposedSize);
    }
}