Tiny midges are an important food source for all trout and, in many locations, they can represent the bulk of what the fish feed on. A lot of tailwaters (like the San Juan River) are almost exclusively a midge fishery, with all of the flies being used size 22 and smaller. Don't think that bigger fish don't eat these smaller bugs - elephants eat peanuts don't they? Available in size 22 and 24.
TIP: In most cases, you will want to use 5.5X or 6X flourocarbon tippet with these smaller flies.