Source: Flying Finger operates its YarnBus, a free shuttle from Manhattan to its store in Westchester, NY.
Yarn, like so many other things (balls, underwear, people, crochet hooks, etc.) comes in a variety of sizes. And with different proportions comes different labels - pool, tennis, baseball, basketball, small, medium, large, extra-large, thong, bikini, brief, boy short, short, tall, fat, thin, A, B, C, etc. Endless labels for an endless assortment of the things that populate our universe.
As for yarn, its size, or weight, is determined by its thickness. Yarns typically fall into six broadly-defined categories of weights: (1) super fine or fingering or sock, (2) fine or sport, (3) light or DK, (4) medium or worsted, (5) bulky, and (6) super bulky. There's also the lace/cobweb/gossamer weight yarns that often are labeled 0. Need visuals? Cobweb (0) -- think of the über-skinny women whose gassamer-thin limbs would be swimming in a size 0 top. Super-fine (1) -- think of the haughty, ahem, God's Gift, ladies for whose anorexic bodies haute-couture is designed. Sport (2) -- think of all the athletic non-running, non-gymnasts, non ice-skating women out there who look as if they can lift a 5-lb weight. DK (3) -- well, you get the point.
According to the Craft Yarn Council of America, specific weights of yarn should produce a somewhat predictable number of stitches when using a particular sized crochet hook or knitting needle. The higher the yarn number, the heavier the yarn and the fewer stitches per inch you will get. But these are just general guidelines. As most crocheters and knitters have found, not all yarns of a certain weight are exactly the same. Thus the importance of swatching. There! I've done my part to encourage folks to take the time to make the dreaded but necessary swatch before embarking on projects where size (stitches that is) does matter.
My personal preference for yarn is driven by the project on which I'm working. Lately, I have have been working in lighter or finer yarns so that I can make garments with better drape. Crochet stitches produce a series of knots that produce bulkier stitches than knitting. So, to make a garment (whether a sweater or a hat) with a soft drape, we need to use a bigger hook, but by doing so, we risk creating a something full of holes. Works great on lace but wouldn't keep a slumbering bear warm on a bitter cold winter night. In general, it would be better to keep using your handy dandy G, H, or I hook and drop down to a skinnier yarn like light/DK and fine/sport weight, although sock weight works wonders. Last year, I made a scarf using a fingering weight washable wool and loved how it turned out. Hats made of any weight yarn will keep your head warm, while I doubt women of a certain age (we know who we are) would be comfortable in a sweater hooked up in a super bulky yarn.
Yet, I also like to quickly work up a hat or scarf, or even, sweater. Pair a big hook with some big yarn, throw in a couple of hours and you've got a hat. A few more hours, a scarf is born. From skinnier yarns can come supple, lightweight fabric does not grow the fabric as fast as bulkier yarns. Slow and steady may win the race but sometimes you need the heady rush of producing something of substance quickly.
Does size matter? Well, yes and no. As with life in general, yarn size just depends on what you make of it.