Okay, here are some pictures. The sweater on the left isn't nearly as bulky as the picture makes it look. In fact, it has a really nice drape to it. It's a pattern from Doris Chan's book Everyday Crochet called Haru. I have no idea what that means. I made the sleeves a little longer than those pictured in the book, and didn't include the side slits at the hip. Well, I would have included them, but by the time I had worked the appropriate number of rows the sweater was already long on me. I could have taken out some rows and reworked them to include the slits, but why? I liked it just fine as it was. The yarn, Simply Shetland's Silk and Lambswool, is just as wonderful as I thought it would be. It's a light weight yarn, but still warm and cozy to wear. I'm very happy with it.
So happy, in fact, that I used some left over in this "waffle weave" scarf where the laciness of the yarn shows up much better. Probably this will go to my sister for her birthday in a few months. Probably.
I like doing scarfs. You can't beat them for perfectly portable projects, and gauge isn't important. Gauge is all about sizing, how many stitches you work per inch. This is important when you are working on clothing because you are usually trying for a particular size, and your sweater, or whatever, isn't going to be the same at, say, 5 stitches psi as it would if you are getting 10 psi or even 2 psi. Too many stitches per inch and your piece is going to run small. Too few per inch and it's going to work up a lot bigger than what you want.
So how do you know? Look at the pattern. A pattern is a set of instructions that tells you more than just what stitches to make. It will also have yarn weight and hook size recommendations, and information on gauge. The hook size is just a suggestion. Here's the thing about handmade as opposed to machine made: stitch size is harder to regulate. Two people can use the exact same yarn and hook, and yet not make their stitches exactly the same size. Back to the original question, how do you know? You make a swatch. A swatch is exactly what you think - a sample. Your pattern will say, for example, '5.5 per inch in pattern.' So you work a few rows using the pattern stitch (double crochets or shells, for instance) to maybe 4" square and then you measure how many stitches and rows you have per inch. If you come out with too many stitches per inch, try again with a larger hook. Too few per inch, try going down one or two sizes in the next swatch. When you get it right, you have your gauge; and when you have your gauge, it's easier to work a garment to the size you want.
When I started the sweater, I think I did three 4" swatches before I got it right. Is it a pain having to possibly work a few swatches before you can really get started? Yes. Which is why something like a scarf or afghan is nice sometimes; no gauge worries. If it's a few inches short or a few inches too wide, who cares? And if I don't tell my sister that the scarf is the wrong size, she'll never know. Nya!