I've had the most success with the 12 to 16 range. A white or gold bead head chironomid with a red/gold body is my go to for the lake trout in the Pacific Northwest. It's also picked up some smallmouth bass that get curious. Sometimes green/brown patterns work well, too. These are for imitating midge larva and pupa - the largest portion of a lake trout's diet.
A couple ideas on the "retrieve" (depending on the lake & conditions):
- Let them hang out under a little indicator. The ripples on the lake give it a tiny bit of action and you have excellent control over the depth
- A slow retrieve that mimics the pupas' rising action is a "tug-tug-tug-paaaause" - very effective if you do this across a long feature like a sunken log
- Drift downwind with the fly trailing behind you, assuming you're on a tube or a boat
You'll also want as light a leader as you can handle and in a generous length (think 10ft+) because stillwater is usually quite clear and gives the fish a long time to investigate. You're not as concerned with a perfect taper that will give you that beautiful cast.
Yes, you're going to get a fair amount of grief from the purist fly fishermen telling you that it's not "real" fly fishing. The truth is that fish do most of their feeding under the surface so why focus on the top?
The other killer pattern for lakes and ponds: leech patterns (wooly buggers, etc), minnow patterns (if you have aggressive brown trout), and mice/frog patterns for big aggressive freshwater species that like to ambush from grasses, etc.