In knitting, sometimes it is very hard to see the whole picture. You have an idea, you might even see a picture of what someone else made. But your yarn is different, your tension is different, your very movement, heartbeat, and breathe will influence the finished object to make it uniquely your own.
Although this sounds abstract, it is absolutely practical and real. For example, a person who pauses to pull the yarn tight with each stitch may fit 6 stitches in an inch of knitting. My loose looping might give me only 5. If the pattern calls for knitting 200 stitches around a sweater, that 6 stitch an inch makes the sweater around 33 inches around, a women's size small. My 5 stitches an inch will make a sweater 40 inches around, a women's large. A huge different from one stitch each inch, a ripple from a dropped rock becoming a wave.
To get an idea of the whole picture, you swatch, make a sample, a trial run. If that isn't what you want, it's much less painful to rip that out and try again than it is to rip out half a sweater. So, to make the knitting fit, you can change things, the size of needles, even the kind of needle (wood or metal or acrylic), maybe even the size of yarn. You can think outside the box, and knit a size small with your loose knitting style to get a size large because you've adapted for your known style. The key is to remember there are options, to know your style, to try a sample, to adapt to make things the way you want.
There are surprises. Even with all this, swatching and planning, things can happen that make the result ... different. For example, the knitting can happen when you are anxious and worried, and your normal loose loops have turned into cramped and tortured knots. Or, the fabric looks great, until it is washed and laid flat to dry with the respect earned by good wool. And that good wool grows 3 inches, bringing to mind sheep with long braids in the rain.
The unexpected is not a disaster. If committing to a sweater's worth of knitting time, one may now learn to swatch and then wash the swatch, too. One may decided to knit only a small cloth in the doctor's waiting room with worry keeping company, instead of a sleeve that no arm will later fit through. One learns. One hides the braided sheep's sweater at the bottom of the bin. Maybe, in a spurt of thrift and resilience, one rips out the old and reuses the yarn more appropriately. Sometimes, yarn knows what it should be, and it is wise not to argue.
The whole picture seems to be made up of lots of little bits of wisdom, made more beautiful from the learning.
And more often, I admit, I don't swatch, I just start. The yarn, the colors, the movement seduce me and I am off, creating and going and hoping for the best. All the bits of wisdom not forgotten exactly, just tucked away, useful still. And sometimes, I get lucky and the unexpected is a pleasant surprise.
May your yarn be soft and happy, may your fingers be limber, and your heart brave. Onward.