The contrabass saxophone is the largest member of the saxophone family ever created. Adolphe Sax did have plans for a subcontrabass, but the closest we have to one is the Bb subcontrabass Tubax made by Benedikt Eppelsheim.
Their Tubax, however, has physical differences from the typical saxophone's bore shape to make it more compact, but these qualities make it separate enough that it is often considered separate from the saxophone family.) It sounds a full octave below the baritone sax, and is also considered an Eb instrument. The range seen on a Contrabass part is usually from A3 or B3 to F6 or F#6 on a treble clef, or concert pitch C1 to A3. It has the same fingering as any other standard saxophone. The mouthpiece used is often a bass saxophone mouthpiece. The average contrabass can weigh 45 pounds or more. Therefore, most models have an adjustable peg (just like a bass clarinet and other large woodwind instruments) added to the bottom curve to help with balancing the horn for standing or sitting. Due to the high cost of the instrument (often over $20,000 USD!), few players invest in and master this horn. It is almost completely in the realm of professionals. Some players who play the contrabass sax are:
To hear a great example of the contrabass in a jazz group, listen to Randy Emerick play "Stardust" below: