Size adjuster (10) for helmets for cycling use of the type that can be fixedly connected inside a helmet for cycling use and comprising an annular structure (11) arranged along the lower periphery of said helmet and able to be switched between a minimum diameter configuration and a plurality of increased diameter configurations to allow said helmet to be worn by different sized users, foreseeing a manual switching device (12) of the configuration of said annular structure (11), characterised in that it comprises headrest elements (13) for a user connected to said annular structure (11) in a freely orientable manner along at least one axis.
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