I have recently found comfortable positions, toys and places to be able to really enjoy anal stimulation while I masturbate. Generally i'll use either a butt plug or dildo. I've tried toys with vibration settings on them and I find they only pleasure when used on the hole, not in it.
I've tried small, medium and large toys while masturbating and have found i enjoy mostly the medium range. So around 1 1/2 inches diameter, easy to get in and move around with. I did recently have a larger 2 1/2 inch dildo in me the other day while masturbating. I was riding it gently but found that when I came that it took away too much from masturbation. Something that large was more of a distraction.
While plugs are fun, overall i prefer riding a silicone dildo medium size, especially in the shower. I can take care of business, clean the toy and myself straight away. Perfect.