The average size of a prison cell for one inmate is 6 x 9 x 12 feet. Generally, that space includes a bed, a sink and a toilet. In the USA, approximately 80,000 prisoners are held in cells of that size by themselves; a situation known as solitary confinement.
Prison cells that hold more than one inmate vary greatly in size depending on the state and facility. For example, Texas prison standards require a minimum floor space of 40 square feet for one inmate, and 18 square feet for each one thereafter, up to 24 inmates. In that space there should be seating sufficient to provide not less than 24 inches across for each inmate. The space should also include at least one toilet and one water fountain.