Paul,
Streamer size is a fascinating topic - I tend to agree with Jay and the others that BIG is GOOD. This is especially true for bass. But, as with so many things, you can get diminishing returns as castability starts to degrade. With due respect to Kelly Galloup, I disagree on placing the hook close to the head. This is fine for bass but my experience is that trout (and pike and muskies) hit the back part of the fly and I miss a lot of trout on long tailed streamers. As for size, I am going to take a different road with regards to describing size. When folks describe streamers by hook size this is a poor referance as the wing can extend way beyond the hook. It is better and more accurate to describe streamer sizes in inches from eye of hook to tip of tail. Once you have a notion of the length one can imagine a matching hook size.
In any case, I consider any streamer for stream trout that is over about 2.5" to be large. I like "monster streamers" for big waters in the fall - these would be articulated trout streamers from about 3" to 5". Since you're interested in smaller streamers, I'd try a bunch around an inch to 1.5" in length. These are good imitations of large swimming nymphs as well as YOY baitfish common in small sizes in the spring and early summer. Although I tend to like big streamers, most of the sculpins I fish are actually pretty small, in the 1.5" to 2" range. I also like a small streamer for mountain brookies, these would be flies about an inch long. Small streamers catch a lot of fish and are better hookers IMO. Of course, if you're after that trophy fish and don't care to catch average trout, stick with the big stuff.