We recommend the following sizes:
½ scale guitar for kids between 5-6 years old
¾ scale guitar for kids around 7-10 or 11 yrs old
Full-scale guitar for ages 12 and up.
These are averages, so if your 6 year old, for example, is on the taller/bigger side, they might do well with a ¾ scale guitar. It’s best to try out a couple sizes at a local guitar shop to make sure you get a good fit.
In addition to scale, there are different body and neck types to consider. Body styles range from smaller "travel" guitars to the imposing "jumbo" profiles. There are obviously no right or wrong styles; it's simply a matter of preference regarding sound and fit. The formula is usually simple: the bigger the guitar, the bigger the sound. Tiny travel guitars have a small, sometimes thin sound, while the dreadnought (the name even feels heavy, doesn't it?) can project a full range at impressive volume.
Regarding the sound, though, construction is the x-factor. I've played small, hand crafted guitars that effortlessly projected rich, wide sounds, and I've played cumbersome dreadnoughts that sounded pinched and shallow. Choice of starting materials and care of construction make all the difference. My favorite body styles tend to be concert (or "grand concert") or auditorium, but again, it's just a preference. (Be sure to see This Article for guidance on guitar brands and pricing.)
Regarding sizing for younger students, I would recommend erring on the side of smaller rather than larger. It's a discouraging scene: a student comes to their first lesson giddy and motivated, but then spends the next half-hour wrestling their oversized guitar into playing position! It's just one additional (and rather bulky) hurdle between them and success.
Many students are also encouraged to start with the classical guitar method...which in turn recommends a classical guitar...which happens to boast the widest and thickest neck anywhere in the history of space-time. Classical guitars typically have nylon strings which are gentle on tender finger tips, and this is sometimes why they're recommended for beginners. But what does that matter if your fingers can hardly reach the top three strings anyway? (And besides, you can put nylon strings on any acoustic guitar, classical or otherwise.)
The bottom line? Find the appropriate scale, body, and neck type, lean smaller rather than larger (at least when starting out), and you or your child will feel confident every time you pick up your guitar!