Basic leader lengths range from 4 or 5 different lengths. The shortest one is a 6 foot leader. These are mainly used with sinking lines to keep your fly close to the bottom because nylon tends to float, and with a long leader your fly would get too close to the surface. So, use a 6-footer for sinking lines which is a rare occasion in most trout fishing.
The next size is a 7-and-a-half foot leader, and a 7-and-a-half foot leader is pretty much your basic small stream leader which is what I would be using in a stream like this. The fish aren't spooky in here, the water is fast and I'm not going to worry too much if my fly line lands pretty close to the leader because the fish aren't going to notice it. And 7-and-a-half foot leaders work better with short casts and most of my casts here are going to be short. The longer the leader, the tougher it is to turn it over to get it to straighten with long casts. So a 7-and-a-half foot leader in a situation like this is about right.
If I were to go to a little bit bigger river and the water was a little flatter and the fish were a little spookier, more suspicious, I'd probably go to a 9 foot leader. That's kind of the standard leader, if you don't know what lengths leader to buy, a 9-footer will get you by in nearly all circumstances. That's the basic trout leader you use in most rivers - the 9 footer. So if you remember nothing else, remember 9-footer.
The 12 foot leader, as the water starts to get lower in the summer or you get really low clear water and every time you make a cast, the fish stop rising or they spook or they run away, you probably want to go to either a 12 foot leader or under really extreme situations where the fish are heavily fished, the water is clear, they see a lot of fisherman, they see a lot of flies, you might want to go to a 15 foot leader. Now 15 foot leaders and 12 foot leaders are a little bit more difficult to straighten, however, you're usually casting a longer line and the leader, well, if it's well made, will straighten out just fine.
The other thing you want to think about, is you could get a lot of wind. The more the wind blows, the shorter you want to make your leader. And you could probably get away with a shorter leader when it's windy because the surfs in the water will be more rippled.