Similar to the other instruments in the string family, cellos must be measured to fit their players.
Often the teacher will measure the student and make a recommendation for the appropriate size. If your teacher recommends a size, that is the size you should get regardless of any measurements you or anyone else might do.
If you're figuring out the correct cello size on your own, follow these steps:
1. Have the player sit so their knees are bent at 90 degrees
2. Adjust the height of the cello so the left knee touches the curve below the lower bout corner.
3. If the cello fits correctly, the upper rim of its body will rest on the sternum (breast bone) and the lowest tuning peg will be near the left ear.
4. From this position, the cellist should be able to comfortably reach both ends of the fingerboard.