The easiest way to measure hackle is to fold the feather over the hook shank. Before you tie anything on your hook, grab your hackle feather by the tip in one hand and the butt in the other. Bend your feather over your hook pulling up on both the tip and the butt. The barbs will stick out away from the hook shank and you will be able to see how long they are. Kind of as a general rule, especially for a beginner, you want the length of the barbs to be one to one and a half times the width of the hook gap/gape. The gap is the distance between the point of the hook and hook shank directly above it. Sometimes wooly buggers have the hackle tied in at the head and wrapped back to the bend and then a wire rib wrapped over it to hold it in place. For a beginner, I would recommend tying the hackle feather in at the bend of the hook before tying in the body material (chenille, peacock herl) by the tip and then spiral wrapping the hackle feather to the eye (or in your case the bead) and tying it off with thread. A wooly bugger does not need a wire rib. When you tie in by the tip and wrap toward the eye, you will get a natural tapper because the barbs at the tip are generally shorter and the barbs at the but a little longer. That way your longer barbs are further away from the hook gap and less likely to crowd the gap.
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