This following excerpt is from Dominican Spirituality:  Principles and Practice by Fr. William Aquinas Hinnebusch, O. P. and gives an account of our knowledge of the physical description of St. Dominic from a Dominican Sister named Cecilia.  Blessed Cecilia was a young woman, not yet twenty, when she received her Dominican habit from St. Dominic at Rome and witnessed the foundation of the monastery of San Sisto. In 1223 she went with three other sisters to the new monastery of St. Agnes in Bologna to help train Blessed Diana and her community in the Dominican life. She remained a member of the Bolognese monastery until her death. Among the treasures Cecilia brought from Rome were her recollections of the Holy Founder. Late in life, some time between 1272 and 1288, Cecilia dictated her reminiscences of St. Dominic to Sister Angelica. This was the origin of the book The Miracles of Sister Cecilia, from which we get our description of St. Dominic.

"Of all those who knew the founder, Sister Cecilia alone describes his physical characteristics and appearance. At the very end of The Miracles there is a fine verbal portrait of St. Dominic. Modern information helps to substantiate what she said. After World War II, Pope Pius XII authorized the Dominicans of Bologna to have the relics of the founder examined. During the war they had dismantled the tomb and placed it with the wooden casket containing the relics deep in the basement and covered them over with sandbags. After the war, with the Pope's permission, the Provincial of Lombardy had the relics examined by X-ray. He was not permitted to open the casket, but photographs from many angles were taken. Almost all the bones are still there after more than seven hundred years. Doctors and anthropologists were able to study them and give an accurate description of the skeleton and physical characteristics of St. Dominic. The Pope was so pleased with the results that he allowed the opening of the separate reliquary containing the head of the Saint so it could be examined more carefully.

"For a long time historians did not think very highly of Sister Cecilia's memoirs. Their reasoning was that she was too old when she dictated them; she must have exaggerated all that she said; much of what she said seemed far-fetched; she must have given her imagination free play. But the study of the relics gave the lie to these doubts, at least so far as what she said about St. Dominic's appearance. Her description is proved reliable by the scientific examination. She said he was of medium height -- the measurements show that he was five feet six inches tall. She said, 'his figure was supple; his face handsome and somewhat ruddy; his hair and beard blond with a reddish tinge. He was not a bit bald, though here and there in his hair there was a touch of gray.' At the bottom of the reliquary, the examiners found some shreds of St. Dominic's hair. It was exactly the color that Cecilia had said it was. 'From his brow and eyes,' she continued, 'there came a radiant splendor which won the respect and admiration of all; his eyes were large and beautiful.' St. Dominic's remains show large eye-sockets that are widely placed, confirming the physical description of Cecilia. With the scientific measurements and Cecilia's description an artist has reconstructed an image of St. Dominic. At least in size, shape, and proportion it conforms to life. Cecilia added: 'His hands were long and handsome and his voice powerful and sonorous, and he was always joyous and smiling, except when moved with compassion at the affliction of his neighbors.' There are very few saints of so long ago whose personal appearance is so well described."