Size 1 of model 710-1 is intended for a neck height of about 6 cm, which makes it appropriate for a child rather than an adult. Getting this height right matters: the collar should hold the cervical area in place without pushing the head up too high or resting too low on the shoulders.
To check the size, have the child sit upright and measure from the angle of the jaw down to the top of the collarbone. If that distance is close to 6 cm, size 1 is the likely fit. When the collar is on, it should sit snugly under the jaw and rest evenly on both shoulders, keeping the head in a neutral position without forcing it. Normal breathing, swallowing, and speech should remain easy.
A simple daily check: you should be able to slide one finger between the collar and the front of the neck. If the child can twist the head freely, the collar is too loose; if the skin is pinched or breathing feels blocked, it is too tight. In either case, stop using it and ask the specialist who recommended it.
Children grow quickly, so re-measure the neck height every few weeks. After each use, look for red marks, chafing, or dampness under the collar; these can mean the fit has changed or the collar is being worn too long. If there is any doubt, let a professional confirm that size 1 is still correct.