I asked this question (in a private message) to one of our esteemed members, Jds4mds, who has actually taught the procedure in medical school. He described in great detail how to do it. He said he never taught a person whose fingers were too short to reach the prostate. That was the basis for my question since I wanted to teach my wife how to do it and she has short fingers. My wife and I have done his procedure and it does work. I should say it works really well when my wife works my prostate with one hand and jacks me off with the other!
So here is a direct quote from Jds4meds (I hope he won't mind me sharing)
Yup, I teach this. I'll answer your second part first. My students are students, usually 2nd year med students, sometimes advanced practice nurses or sometimes physical therapy or pharmacy students. No matter what flavor, they've all been through anatomy class, so they know the basics. My class is practical. I'm the body they practice on, as well as the lecturer. There are usually 3-6 students in a group. I explain the procedure, including positions and procedures. During practice, I guide them verbally (sometimes physically for the genital exam). Basically, I have multiple DREs in a session. Yes, I can feel if they're on my prostate and if they're too far left or right.
In the hundreds of students I've taught, I've never found anyone who couldn't reach my prostate. That said, I'm sure there's some combination out there of short fingers and deep prostate that just can't work. My advice to them is that you can't change nature - you can only do what you can do.
Technique is usually the key. When you examine a prostate, it lays between the rectum and the belly button. Keep that direction in mind regardless of the position of the guy. My suggestion is to start simple, either bend over a table or get on your hands and knees. Your wife applies lube to your external anus with her dominant index finger. She makes a toy gun with her hand. She inserts her finger with the pinky side of her hand towards the floor. She inserts her finger to the hilt then rotates her hand so her palm is down. The prostate should be directly under her finger pad. Sweep side to side to feel it. It has two sides with a depression in the middle. If she's not finding it, have her make sure that her shoulder is right on your midline, otherwise she's probably pointing off to the side.
You'll know that she's right on because you'll feel strange things. You might feel like you're peeing. You might feel strange sensations in your penis, especially right at the urethral meatus (pee hole).
Just make sure she has very short nails. You don't need damage. Also, don't do this for about a week before a PSA blood test, you'll get a high reading for sure. I had that scare and learned my lesson.