Since we are talking brook trout...
Here is the way I look at it.
For brook trout I rarely use flies smaller than a 14, and almost never smaller than 16. Brook trout are not picky where I have fished for them. It's more important that they don't see you, than it is to match any hatch. Chunky stuff works.
IMO (when talking trout) rod "weight" has more to do with what flies you will be using, not what fish you are trying to catch. Lighter weight rods are great for lighter weight flies, so unless you are casting midges and things like that to brook trout, you don't need a super light rod. Rod weight doesn't matter if the trout is 5 inch or 12.
Johnw and maybe a couple people mentioned over-lining. I think this helps and John explained why. In the tiny brook trout streams I used to fish (It's been several years), you aren't going to be doing a lot of back casting, but you will likely be doing some roll casting and flicking or shooting the fly short distances. I don't know if I am using correct terms or not. Whatever it takes to get the fly in front of the trout without spooking it, and it often does not permit a back cast. Over-lining helps.
Also, I think rod action is important when it comes to this type of fishing. IMO, a more moderate action rod is going to "work" better than a fast action (IMO). St. Croix makes great moderate action, which explains why reference johnw's response. If I were to buy a new, reasonably priced rod, specifically for this type of fishing, it would be something similar to what johnw is using.
Then again, I never was a fan of fast action rods.
Others might disagree, which is fine. This is just opinion.