The overall length of the guitar is not a good indicator of whether it is the correct size for the student. The only way to know the true size of a guitar is to measure it's "scale length". The scale length of a guitar is measured from the "bridge" of the guitar to the "nut" of the guitar. See the example below
As you can see from the picture above, the scale length measures the part of the strings that actually vibrate when you pluck the string. This is the only relevant way to compare guitar sizes. A 38" classical guitar and a 41" steel string guitar can have the exact same "scale length" and both of them can be "full size" guitars. The only way to be certain what size they are is to measure the scale length.
