I've had two types of prison in the past.
Most have been for people accused of production or export related crimes by nobles completely removed from reality. Since the prisons were built just to keep the noble happy, and since the prisoner generally was completely innocent, I made them nice 3x3 rooms with a high value chain, a bed, table & chair, plus separate 1x2 stockpiles for booze and food.
The last time I built a prison, however, it was for a vampire who had murdered several of my dwarves. He was my mayor and when I realized he was a vampire, I put a lever in a remote room, set it for only him, and put it on repeat. Then I locked the door once he was in there. Fast forward several months and I noticed that I had the liaison from the MountainHome hanging around my dining hall. I realized he wanted to meet with the mayor and couldn't, so I released the mayor. The liaison went to the mayor, the mayor's job changed to "conducting meeting" and he headed for his office, part of his suite of room in my bedroom level. I was watching him on the way when he suddenly stopped into a bedroom with a sleeping dwarf, and with the liaison standing there began to feed on the sleeper. So I ordered a militia squad to the room. they showed up and proceeded to stand around watching while the mayor finished draining his poor victim. Sheesh. At least they reported the crime and I had plenty of witnesses, but you'd think they might, you know, do something to stop the vampire?
So I convicted the mayor for that murder. But since I didn't have a prison set up, the sheriff just came along and slapped him around a bit. No lasting effect.
So I built a chain just outside the entrance to the fort and designated it as a prison. Then I convicted the mayor of every outstanding vampire murder, which gave him a nice long prison sentence. The next siege to come along solved my vampire problem.