The size of a fly fishing fly is, as you have noted, indicated by a number. The smaller the number, the LARGER the fly. Most trout dry flies run in sizes from 12 – 18, although midges can go down as small as 22 (a real eye-strainer when you are watching them float down stream). Streamers may run larger for trout, say a size 6 or 8 wooly bugger. Your best bet for selecting the right size is to find a book or website that has a hatch table. Most of those will give you the size of the fly that most closely approximates the size of the bugs in that hatch for a specific region during a specific time of the year. A good example of this the "Trout Stream Insects – An ORVIS Streamside Guide" by Dick Pobst. You should also check out the Field Guide "Guide Books" posted on Cabelas.com. Those provide some very helpful information for the starting fly fisher. Good luck and tight lines!
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The size of a hook is determined by the distance of the gap between the point of the hook and shaft of the hook. The length of the shaft does not affect the indicated size of the hook. So, you could have a #16 hook that is the same length as a #12 because the #16 may have a 2X or 3X long shank. I know is sounds confusing and sometimes it is when it comes to buying hooks.

The weight of the fly has nothing to do with the indicated size of the fly.

But, you only need to think of the size of the fly as based on the gap size, in other words, is it a larger size #10 or #12 or #14 or #16, or a smaller size #18? etc. The people who tie the flies are the only ones that need to be concerned with shank length or the thickness of the metal of the hook.

As for the size you need for trout fishing, that really depends on the water you are going to be working. You can find that out by calling a local fly shop, if there is one around, or by reading a book related to that body of water you are fishing.

Fish tend to key in on insects that they are used to, with some exceptions. Some parts of the country have very small insects and others have large insects. I was recently fishing for steelhead in the rivers of the North Shore of Lake Superior in Thunder Bay, Ontario. In talking with the local fly fishermen there, they seldom go below a size 12 fly, often they use 10's and 8's, because the fish key in on larger flies. When I fish the Missouri River in Montana, most of the time we use size 16 and 18 flies, because those fish key in on smaller insects.

Like I said, there are the exceptions. For instance, what fish ever saw a Chernobyl Ant? Yet they will take that fly sometimes when they pass on many others. However, when fly fishing, it is best to stick to the usual suspects of flies and just have a few odd balls around for when all else fails.

I prefer to have several sizes of the same fly with me when I fish. For instance, I have the Parachute Adams in size 12 & 10 & 14 & 16 & 18. It is such a great fly to fish with and I prefer to have all the sizes to try to match the hatch size. Usually, you only need to have 2 or 3 sizes of any one particular fly, for sure at least 2 sizes.

When fishing, try to match the size of the fly to the size of the tippet. Don't use a 3X tippet on a size 16 fly. Also, don't use a 7X tippet on a size 10 fly. The larger the fly the more tippet strength you need to turn it over properly. Naturally, since this is fly fishing, there really are not rules set in concrete. Sometimes you have to use a larger tippet because you are going after a much larger fish that is keying in on a really small fly. However, usually, when a fish becomes larger it is more of a predator and less leader shy, as a rule of thumb.

Hope this helps.

Larry
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