The space measures 9 x 5-3/4 approximately. It's the same machine as the Singer W9 (or 9W?) that was made with the leftover Wheeler & Wilson No. 9 parts when Singer bought the company in 1904. I think the picture gives an optical allusion of great height, and I think it is a bit higher than standard. It was advertised as a "High Arm."
I'm guessing it's great for quilting, and I would love to hear from other owners of this treadle machine.