At age 15, Paul Atreides was tested by the Bene Gesserit to test whether he was human:
From the folds of her gown, she lifted a green metal cube about fifteen centimeters on a side. She turned it and Paul saw that one side was open - black and oddly frightening. Paul slowly put his hand into the box. He first felt a sense of cold as the blackness closed around his hand, then slick metal against his fingers and a prickling as though his hand were asleep...
Reason told him he would withdraw a blackened stump from that box.
Youth anthropometry measurements place the 50th percentile of 14.5 to 15.5 year old males as having a hand length, from the start of the palm to the tip of the furthest finger, at 18.2cm. The 5th percentile was 16.3cm. You have to go to a 10 year old before the 50th percentile is below 15cm, at 14.9cm.
Adult male hands, at the 50th percentile, at 18.4cm in length, and would have to essentially form a fist to fit inside a 15cm cube.
The book doesn't describe him forming a fist, and the "blackened stump" suggests that the hand up to the wrist entered the box.
Is there any information or other measurements in the books which suggest that Dune humans were smaller than us in any specific way?