How big is the spinal cord?

Not very. The spinal cord is roughly 18 inches long and somewhere between 1/4 and 1/2 inch in diameter. It extends from the foramen magnum (the giant hole at the base of the skull) to approximately the 10th-12th thoracic vertebrae.
1.5 feet long. The spinal cord measures about 43-45cm or 17-18 inches long. The width is variable, since it has 2 areas of enlargement at the cervical and lumbar region. Thicker regions of the spinal cord measure about 1/2 inch or 1.25 cm (close to the thickness or your thumb) and the thinner areas measure 1/4 inch or 1.13 cm (close to the thickness or your fifth finger).