Many anglers tend to overlook large flies, especially large nymphs in tailwaters. While it is true that the cool constant water of tailwaters tends to produce large numbers of midges, small mayflies, and other invertebrates, there are some big food items in any stream, even tailwaters. Here are a few larger nymph patterns that work, even in tailwaters where the average food item being consumed by trout is size 18 or smaller.
San Juan Worm - This fly was actually made famous on a river famous for its tiny midges. However, most rivers, especially tailwaters and spring creeks, hold large numbers of aquatic worms. The San Juan Worm comes in many colors, even multi-colored, to match the worms in nearly any water. To find aquatic worms in your stream try finding a place with a sand or mud bottom and dig up handfulls of the stuff. It should eventually have a worm or two in it. Even if you can't find aquatic worms, the San Juan Worm works. Probably because it also mimics terrestrial worms quite well.
Bitch Creek - This classic nymph pattern probably originated to immitate large stoneflies and such. However, the Bitch Creek draws strikes, even in streams with no large stoneflies. The bright colors contrasting with black and the wiggling rubber legs make it hard to miss and hard for trout to resist.
Cranefly Larvae - I remember the day that I first found some of these guys on a stream known for tiny insects. It was an eye opener to find bugs an inch and half long that the trout would likely go crazy for. Since then I've kept several Cranefly Larva patterns around and when things get slow I'll pull them out and nymph them way down deep. This fly has produced some of the largest stream trout that I have caught.
The general rule is, when fish don't strike, go smaller. I say don't be afraid to break rules, especially that one, and try some big nymphs sometimes.