Find a series of photos and other sites at http://www.vikingdenmark.com/trelleborg-viking-fortress-slagelse-denmark.html/ If you arrive here late, and the visitor center is closed, you still have access to the grounds.
Were the people small in stature, or was this small door for warmth and protection, much as castles had some very little doors leading to passageways and little staircases for escape. A fully armed and armored fighter could not enter.
In The Vikings, A History, by Robert Ferguson, Penguin Group 2009 at page 10 notes the examination of a Viking-Age man in a grave mound (the buried Tune Ship) in Norway. There were thigh bones thirty percent heavier than those of modern men, perhaps from extensive horseback riding. There was also evidence of a pituitary tumor, however, that could have resulted in some gigantism.
How much is really known about the culture, and how do we know it. I am still reading The Vikings, and am surprised at how late it was before an era of Vikings was seriously identified -- the 19th Century. Issues of how to weigh the old sagas, and how much did the recorders of the sagas, acting centuries later, press their own families' agendas in them. Like now. Text criticism applies to any ancient and modern material.