Interesting thought but again another myth, hieghts in the middle ages where comparable to today, the average height now is only about one inch more than it was then. They had a very good diet, when the crops didn't fail 
I don't really believe that. In the place where I live is the world's largest collection of historical arms and armours (http://www.zeughaus.at, from 24th of February you will be able to see a number of their exhibits including a really beautiful parade armour in the Cleveland Museum of Art if that is anywhere near your place) and the armours that they have there just don't fit a man of today's average size. The breast plates are way too small and only teenagers and some women can lower the nose protectors when wearing a helmet because otherwise the heads are too big for the helmets (I tried that myself last sunday..). The guide said that while the rich could have been about our size, the majority had a much smaller built and he said that the main reason was malnutrition.
I have also visited a number of countrymen houses from the 16th and 17th century and they also don't appear to be have been built for 69" people. You have to take care not to bump your head when walking through a door and the beds were so short you would hardly believe they were made for adults.
We tend to think these days of a linear increase in just about everything, kids these days seem to be bigger and cars are faster than they were twenty years ago. Over the last 100years there has been a noticable increase in height, I'm gob smacked at how tall a lot of 18 year olds seem to be these days most seem to over 6ft tall. But as with most things it's the extremes you notice.
Height throughout history has gone through many fluctuations depending on nutrition and population density. Studies of the skeletons found on the Mary Rose show average height was 5'8" and studies from 14th Century skeletons in Poland give the same average. It is interesting to note that it is the same average height as found amongst men in Australia today, in the uk I believe it is very slightly higher.
As for houses, doorways are no indication of how tall a man is, it is more an indicator of his wealth. A poor man could afford a smaller place and to heat it cost money and effort, bigger doors led to more heat loss, higher ceilings meant it took longer to heat. My wifes sister lives in an early 15th century house, the original cieling levels can be easily seen and they are much higher than those there today. Does that imply that the original owners where all over 6' 6", no of course not. It merely shows that it was a wealthy household and could afford a large spacious house.
It is also known that heights dropped from the 1700's onwards as society changed from a rural to urban based one, whilst it is true that an average man today would stand higher than someone in an industrial revolution mill it is also true that a 15th Century male would on average look down on that mill worker.
I have a good friend who is studing for his phd in medieval history, specialising in military defences and as part of that he worked at the Royal Armouries, trying telling him that everyone in the medieval days was 5' 2". Sorry that sounds flipant. He illustrated the point by showing me a photo of a collection of armour in I believe Italy. Looking at it it looked odd, one suit of armour was huge and all the others looked small, he then pointed out that most of the smaller suits would be worn by men standing about 5' 10" which put the larger suit at about 6' 6"
