The detail on page one that most strongly suggests Beowulf's power is Beowulf's size compared to others.
Beowulf's power is that he possesses superhuman strength. He shares the appearance of the first monster that he killed. This hero is strong and courageous and is described as having the strength of 20 men in just one of his arms.
Page one gives details as to the enormous size of Beowulf compared to others, which allows readers to infer that a key point of his powers may be a great strength.
