I have since fleshed Eileen out physically with chestnut hair, a good figure and great skin. But I do think the physical attributes are less important (though Eileen has been known to use her physical wiles to help her cause, especially when she was living in the Kingdom).
Had I been stuck for physical attributes, one good idea often repeated in writing circles is to take photos from magazines and so on and “blend” different features from pictures you like to create your heroine.
I’ve found it helpful to “see” my heroine in action, doing things, speaking (and getting a good idea of her voice did help give me her class and as a result ideas as to what she’d look like – she’d be unlikely to be a blonde bimbo) and from that other aspects of Eileen’s life, including her looks, fall into place.