Bullhead, the crappie aren't helping your bluegill size any; probably they're hurting it, as well as the bass size for that matter. Crappie are bad bad bad for ponds. There are diverse opinions on here on just about any other subject you could think of, but it has been my observation that we're all in agreement on this. There are multiple threads on here on the topic if you're in the mood for some sobering reading.
It's almost a sure bet your bluegill are overpopulated if you've never caught one over 5". If you're thinking about remedying the problem, the first thing you should do is install at least one automatic feeder; if the bluegill weren't overpopulated you could go with one feeder per four or five acres, but the way your pond is right now you won't get much growth short of putting at least one feeder per acre, preferably two. Feed with Purina Aquamax 500; it's higher protein than most other foods out there, and is far better in terms of ingredients since it's mainly fish protein rather than plant, which catfish chow usually is. Catfish grow well on plant protein; bluegill will grow on it, but nothing like they will on fish protein.
Next, if you want bigger bluegill, stock some more bass. They'll thin the bluegill out so the ones that live have more to eat and the survivors will grow vastly faster, and get vastly bigger. If you have great numbers of bluegill you could also consider stocking a few esocids, either northern pike or tiger muskie, five or six per acre. They'll not only thin out the bluegill population to a better balance - and thus vastly faster-growing bluegill - in short order, but unlike the bass-heavy method of growing trophy bluegill, the esocids will also keep your young bass from overpopulating so the bass still grow well. You wouldn't want to stock them in a pond under one acre, but they've been successfully stocked in ponds that size and up, and resulted in faster growth rates for both the bluegill and the bass. Someone else on here will probably be along in about five minutes to dismiss this idea, but I've personally done it; there's always the chance my results were a fluke, but I ended up with one of the best ponds I've ever seen, with huge bass and bluegill both. I'll have more data from several more bodies of water in a couple or three years on this method.
As to genetics, you're in luck there by geography. You just happen to live in the same state as a man who happens to be on the forefront of growing trophy bluegill, i.e. the mad scientist we all know as Bruce Condello. He has his own substrain of northern-strain bluegill (i.e. what naturally occurs throughout most of the U.S.) that he's been refining for ten years for the fastest growth, best response to pellet food, best hardiness, etc. and has caught bluegill of said strain in his own ponds, over two pounds. You couldn't do better than to buy a few of his 'gills and introduce them to the underachievers in your pond.
