Most knitters enjoy knitting scarves, and a lot of us started with knitting a scarf as our first project.
Scarves are easy because they are usually straight and have a repeating pattern across their length. They're also a great first design project for that same reason.
Scarves don't have to be a particular length or width to be successful; in fact, for that first scarf you may well have just knit until you ran out of yarn, or you get tired of knitting Garter Stitch, whichever comes first, which is a fine way to do it.
But if running out of yarn isn't a consideration, how long should your knit scarf be?
The short answer is: as long as you like. I've made scarves that are very short, and some that are very long, and both have their purposes.
There are some scarves, like the Razor Shell Scarflette, that are really short but are also effective at that length (in the case of that one, it's cashmere, so it's warm even though it's short and you don't want to spend the money to knit a five foot long scarf in cashmere).
This is a scarf you can tuck into your coat and every inch of that yarn is helping to warm you (it's also a one-skein project, making it great for gift-giving, and it's a luxury that isn't too expensive).
And some scarves, like the famous Doctor Who scarf, which is usually worked to 12 or 14 feet long, are longer (and wider, for that matter) than could be considered practical to the knitter or the wearer.
In general, however, a scarf needs to be long enough to wrap once around the neck and have some length left over to tuck into a coat of leave dangling as desired.
I remember once hearing the tip, possibly from Vickie Howell but it was so long ago I don't really remember, that a good length for a warm scarf is roughly the same as the height of the recipient.
I try to use this rule of thumb generally when I'm designing warm scarves and shoot for around six feet in length (I'm not six feet tall, but it's a nicer number than five foot six).
Of course when designing for children, or adults who are either very tall or very short, you can make shorter or longer scarves as you like. And sometimes a really long scarf is a lot of fun, no matter how tall you are. (But for children, an extra-long scarf can be a safety hazard. You might even consider knitting a cowl instead of a scarf, so the child can stay warm without scarf tails that could get caught in things.)
What About Width?
Since I mentioned it, you may also be wondering if there's an ideal width for a scarf.
Again, it depends on your preferences somewhat. Sometimes a skinny scarf is fun, or nice for when the weather isn't too cold, but a wider scarf is warmer for the winter.
I say it somewhat depends on your preferences because it also depends on the stitch pattern you are using to some degree. If you're knitting a scarf using a lace pattern, for example, that has a multiple of, say, 18 stitches, you need to take that into consideration when planning your scarf.
Will a single repeat of the pattern look good?
If not, is 36 stitches too wide for the kind of scarf you're envisioning?
This is where knitting a gauge swatch can be super helpful, because you'll actually be able to see what the pattern looks like in your chosen yarn, how it drapes and whether it looks like it will be too wide or narrow as a scarf. The good news is, if you don't like your first choice, there are many other stitch pattern options out there.