There are several aspects of anal toys. Width: which you need to practice over time to be able to slowly increase your ability to take wider and wider toys. Depth: which can be more easily achieved up to a point vs. width given the toy will hit a point of resistance when the tip end leaves the rectum and starts to hit the initial curves of the colon. For a man, applying pressure to his prostate can be very erotic and pleasurable but you don't need either large wide toys or to penetrate deeply into the ass to hit the prostate.
I have done deep penetration into my colon and I would say it is not necessarily erotic but just fun to see how far I can work my large colon tube up my colon. I feel and see it moving under my skin when it begins to move up the descending wall of my colon but for me that really doesn't create any direct erotic feelings. All of the erotic feelings are in my rectum, anus and prostate.
I would say if you are purely after the erotic sensations, then depth beyond your rectum isn't that necessary.