The inside dimension of this hoop is 5 5/16″ (5.30″, or 135mm)

Actual sewing area depends on brand of machine between to 4 5/16″ to 4 13/16″ (110 to 122mm)

Mighty Hoop has a diverse selection of sizes and matches perfectly with Smartstitch embroidery machines.
Youtube video on adding custom frames to the S-1201:  
Youtube video on adding custom frames to the S-1501:  
Youtube video on separate and keep your Mighty Hoop: