Guitars are built with many different scale lengths from short scale to long. The advantage to a longer scale is that it is louder and produces a bigger tone. You can also play harder because the strings are tighter. The disadvantage is that it’s a bigger reach and hard for most people to play difficult music on. People have injured themselves on long scale guitars like the Rameriz 1A.
Short scale guitars on the other hand are easier to play because of the lower tension of the strings and the smaller reaches. However they tend to have a smaller voice. This can be overcome through amplification, but if you want to fill a concert hall bigger guitars make a bigger sound. Smaller people can play smaller guitars and still sound beautiful. Sharon Isben is a small person and plays on a shorter scale, but that doesn’t stop her from playing a serious repertoire very well. Christopher Parkening is tall and long boned with long fingers and he has always done well with a long scale guitar. The important thing is to figure out what’s good for you.
My concert guitar is a Mattingly with a 666 mm scale. Although my hands aren’t particularly large, I’m able to achieve extension through proper thumb placement and shear force of will. I personally need the bigger sound to create the kind of music I want. Although shorter scale guitars like my Gipsy Kings 650 mm is a lot handier to get around.
Essentially 640 mm is considered a short scale guitar, 650-660 mm are considered medium scale, and anything over 660 mm is considered long scale. These are adult size guitars. Children’s guitars can be much smaller, although few have the dynamic range and projection that you’d want. Some people with small hands and small bodies like the parlor guitars of the 19th century. They’re much smaller but they have a sweet sound because the ratio of scale length to body size creates a different dynamic.
I’ve been referring to classical guitars but the same principles apply to steel string guitars. Steel string guitars tend to have shorter scale lengths than classical, but then again they have wire strings. It’s the torque of the strings pressing down on the saddle that creates the sound. Steel strings exert quite a bit more pressure than nylon. It’s all about the ratios.