It all depends on what technique you are using. My understanding of the thought to use a small hook is that it allows live bait to swim more freely and look more natural. Which is suppose to translate to more bites. It's the same thought behind using the smallest jig possible for walleyes, 1/8 oz and under. As opposed to the bass guys who throw heavy assed jigs
I personally use nothing larger than a size 2 octopus hook fishing for walleyes. I use the #2 hooks to to slip bobber with bigger minnows hooked either in the mouth or behind the dorsal fin. And i use #4 for fatheads. For crawlers under cork i use a #4 or #6. For leeches on a bobber is #4. When i rig leeches i use a #6 or #8 depending on the size of the leech.
The aberdeens are real big now for a technique called slow death. The catch there though is the hook needs a special bend to make the crawler spin. The best place to learn about this one is thenextbite.com (sorry to plug another site Jason)