XS: is the size to choose for ribbing on size 20 or smaller.
Small: is an all-purpose size capable of filling many fly tying needs.
Brassie: is an ideal choice for wire bodied flies sizes 18-22 and for ribbing on most trout sized flies.
Medium: is a good choice for ribbing on size 8-12 nymphs.
Large: is a popular choice for streamers and/or large stonefly nymph patterns.