Paper mentioned by John Hawks: "Reconstructed stature of 1.84 m falls at about 99% of Medieval heights [. . .] Thus, tall stature indeed could have contributed to the success of “Charles the Great” as a king emperor and soldier."
Below, a couple other papers that appeared in Hawks's citeulike feed.
The biological standard of living in Europe during the last two millennia: "We find that heights stagnated in Central, Western and Southern Europe during the Roman imperial period, while astonishingly increasing in the fifth and sixth centuries."
New Light on the "Dark Ages": The Remarkably Tall Stature of Northern European Men during the Medieval Era: "Based on a modest sample of skeletons from northern Europe, average heights fell from 173.4 centimeters in the early Middle Ages to a low of roughly 167 centimeters during the seventeenth and eighteenth centuries. [. . .] It is plausible to link the decline in average height to climate deterioration; growing inequality; urbanization and the expansion of trade and commerce, which facilitated the spread of diseases; fluctuations in population size that impinged on nutritional status; the global spread of diseases associated with European expansion and colonization; and conflicts or wars over state building or religion."