Sarah, A lot of it is personal. The best way to figure your length and width is to measure from your own samples. That being said, to get "Alexander" and a couple of motifs in a bracelet, I would probably have to use sewing thread, or something else very fine like #80 tatting cotton or 2/60 silk weaving thread. I don't know the relative size of the size 8 crochet cotton.
Which technique are you using? What kind of lettering? Some of them compress length-wise better than others. Michael
