Today, his feast as a beatus (which, while popular in origin--he was canonized by someone who turned out to be an antipope--was tolerated and perhaps even confirmed by Benedict XIV and, while presumably not infallible in the manner of modern canonizations, is of sturdier legal provenance than saints like Bl. Christina the Astonishing, never officially canonized or even beatified) is celebrated at Aachen. [UPDATE: It appears he is celebrated at Aachen and Frankfurt as St. Charlemagne, not beatus, though in nearly every reference book I have seen lists him as Blessed, perhaps based on the privaye opinion of Benedict XIV. See comments.] For all his private faults, he was a great and Christian ruler--this man pretty much invented Europe, when you get down to it, though in a rather different form than we know it today. Aptly, or perhaps ironically, The Economist's column on the goings-on of EU diplomats in Brussels is named in his honor.

From the Catholic Encyclopedia (1913 edition):

While enough has been said above to show how ready he was to interfere in the Church's domain, it does not appear that this propensity arose from motives discreditable to his religious character. It would be absurd to pretend that Charlemagne was a consistent lifelong hypocrite; if he was not, then his keen practical interest in all that pertained to the services of the Church, his participation even in the chanting of the choir (though, as his biographer says, "in a subdued voice") his fastidious attention to questions of rites and ceremonies (Monachus Sangallensis), go to show, like many other traits related of him, that his strong rough nature was really impregnated with zeal, however mistaken at times, for the earthly glory of God. He sought to elevate and perfect the clergy, both monastic and secular, the latter through the enforcement of the Vita Canonica or common life. Tithes were strictly enforced for the support of the clergy and the dignity of public worship. Ecclesiastical immunities were recognized and protected, the bishops held to frequent visitation of their dioceses, a regular religious instruction of the people provided for, and in the vernacular tongue. Through Alcuin he caused corrected copies of the Scripture to be placed in the churches, and earned great credit for his improvement of the much depraved text of the Latin Vulgate. [...]

From boyhood Charles had evinced strong domestic affections. Judged, perhaps, by the more perfectly developed Christian standards of a later day, his matrimonial relations were far from blameless; but it would be unfair to criticize by any such ethical rules the obscurely transmitted accounts of his domestic life which have come down to us. What is certain (and more pleasant to contemplate) is the picture, which his contemporaries have left us, of the delight he found in being with his children, joining in their sports, particularly in his own favourite recreation of swimming, and finding his relaxation in the society of his sons and daughters; the latter he refused to give in marriage, unfortunately for their moral character. He died in his seventy-second year, after forty-seven years of reign, and was buried in the octagonal Byzantine-Romanesque church at Aachen, built by him and decorated with marble columns from Rome and Ravenna. In the year 1000 Otto III opened the imperial tomb and found (it is said) the great emperor as he had been buried, sitting on a marble throne, robed and crowned as in life, the book of the Gospels open on his knees. In some parts of the empire popular affection placed him among the saints. For political purposes and to please Frederick Barbarossa he was canonized (1165) by the antipope Paschal III, but this act was never ratified by insertion of his feast in the Roman Breviary or by the Universal Church; his cultus, however, was permitted at Aachen [Acta SS., 28 Jan., 3d ed., II, 490-93, 303-7, 769; his office is in Canisius, "Antiq. Lect.", III (2)]. [...] Except on his visits to Rome he wore the national dress of his Frankish people, linen shirt and drawers, a tunic held by a silken cord, and leggings; his thighs were wound round with thongs of leather; his feet were covered with laced shoes. He had good health to his sixty-eighth year, when fevers set in, and he began to limp with one foot.

From Einhard, Vita Karoli Magni (c. 817-833), Chapter 27:

He was heavily built, sturdy, and of considerable stature, although not exceptionally so, since his height was seven times the length of his own foot. He had a round head, large and lively eyes, a slightly larger nose than usual, white but still attractive hair, a bright and cheerful expression, a short and fat neck, and a slightly protruding stomach. His voice was clear, but a little higher than one would have expected for a man of his build. He enjoyed good health, except for the fevers that affected him in the last few years of his life. Toward the end he dragged one leg. Even then, he stubbornly did what he wanted and refused to listen to doctors, indeed he detested them, because they wanted to persuade him to stop eating roast meat, as was his wont, and to be content with boiled meat.

From Wikipedia:

The physical portrait provided by Einhard is confirmed by contemporary depictions of the emperor, such as coins and his 8-inch bronze statue kept in the Louvre. [His] description of Charlemagne's height (6 feet 4 inches, or 193 centimeters) was not far off. Though it was Herculean stature, particularly in a period in which people were a little shorter than most today, archaeology has confirmed his tallness: in 1861, Charlemagne's tomb was opened by scientists who reconstructed his skeleton and found that it indeed measured 74.9 inches (190 centimeters).