What does six feet square mean in calligraphy?

Six feet square means a square with a side length of six feet. You're talking about Dou Fang. I called the one at the bottom. A square picture is a square.

According to national standards,

Six feet fully open: 180 x 97

Six feet three inches wide: 97 x 60

Six-foot couplets: 180 x 49

Six-foot barrel: 97 x 90