If you insert your finger, you can feel your prostate and it’s usually only about as deep as your second knuckle on your finger. You don’t need very much length to hit your prostate. A buttplug that curves toward your abdomen and away from your tailbone will usually hit the prostate better than one that goes straight in.
From inside the anus, is the rectum. The rectum is only about 5-7 inches deep and then the GI tract goes through an S-curve into the colon. So, one danger is getting too much length that shoves too hard against the wall of that S-curve. Again, you don’t need much length.