I am a beginner and want to buy a new classic guitar from a online shop. Is there any way to determine the size of a classic guitar, for example, from the code in its name?
Classical (nylon strung acoustic) guitars aren't normally "sized" - they are normally all very close to the same overall dimensions.
However for children or smaller players, shorter neck versions are available - 4/4 for full size, 3/4 or 1/2 or 1/4 for smaller ones. (For sizes see https://en.wikipedia.org/wiki/Classical_guitar)
Another possibility is smaller instruments: such as the Requinto, which is effectively a very small guitar for adult players.
Finally some classicals may be available with a shallower body, for a more compact grip while in use. These depend on the manufacturer, and will be in their catalog.