Per the Piano Book, spinets are generally defined as small vertical pianos usually 36 to 39 inches tall, with an indirect blow action. They are the smallest of the vertical pianos. Next size up is a console piano (40-43 inches with compressed action). Followed by the studio piano (43-47 inches with direct blow action) and then finally the full size upright at 48-60 inches tall with extended direct blow action. As there are no enforced "standards" for piano sizing and construction, these are general guidelines only. You could, for instance, have a console size piano with an indirect blow action.
For a vertical piano, you would generally want a studio or full size upright with direct blow or extended direct blow action.