Hojo is a word expressing an square area with one jo (an unit of length: 1 jo = 3.03 meters) on each side.
It's also used to indicate a room or a construction of that size, 'ho' meaning a square (for example, hofun (square tumulus) and seihokei (square)), and having the length of 'jo.'
Because a jo is approximately 1.67 ken (=10/6 ken) (ken is a measure of length), one hojo is about the size of a four-and-a-half-mat Japanese room (being approximately 1.5 ken on a side).
Moreover, because the hojo was easily built and easily dismantled, it was favored by priests and hermits. The title of the book "Hojoki" (An Account of My Hut), by KAMO no Chomei, originates from the fact that he had written it at a hojo hermitage.
Additionally, as is seen in an anecdote written in "Yuimagyo" (Vimalakirti Sutra), that when Monju Bosatsu (Manjusri) and his followers visited a hojo of Yuimakitsu (Vimalakirti), everyone could enter that small space, in Buddhism there is a way of thinking that the entire space is contained in a hojo. As a result, the word hojo came to mean the construction in which a chief priest lived. After the middle of the Muromachi period, Buddhist statues and images of the patriarch began to be enshrined in hojo, and eventually the hojo served as a hondo (main hall).