Both 33 inch and 36 inch length barrels were made that I am aware of, although John Morrow in his 'Confederate Whitworth Sharpshooters' cites 33", 36" and 39" barrel lengths. As an aside, at the NRA(GB) Museum at Bisley there is an experimental one with a 60" barrel! �
 
Have a look at the collecting pages of www.lrml.org. There are picture and descriptions of a number of Whitworth rifles, including a Civil War sharpshooters rifle.
 
David