DeHaan has always been tall and is very proud to be. Her parents are also tall; her father is 6 ft 6 in and her mother is 6 ft 4 in, but Allyssa outgrew both of them. In middle school, she had already reached 6 ft 2 in and rapidly grew another six inches in just under a year, between sixth and seventh grade. She was 6 ft 7 in as a high school sophomore and graduated from Grandville High School at 6 ft 8 in. Already the tallest female player in Michigan State history when she joined the basketball team, she unexpectedly grew another inch before playing her first season at 6 ft 9 in. DeHaan is not afflicted with Marfan syndrome or any other kind of overgrowth syndrome, but is simply extremely tall. She has a 39-inch inseam, and she wears a men's size 11 (women's 13) shoes. Deeply religious, DeHaan attributes her great height as a divine gift given to her by God.