It's a great pu$$y exerciser because - well - the cat is what the woman has to use to 'hold' it.
Order one, "fit it to you", then while you are holding it {lol} measure the circumference where a harness would fit best for you. If you do strap-on, chances are you may already have a harness to fit. A few of my standard harnesses already fit my feeldoes (the regular and the slim) so I'd say regular feeldoe is a 2 in ring and the slim is a 1 1/2 in. (have never measured though).