A helmet that's the wrong size doesn't protect as intended, no matter how good the helmet itself is — fit matters as much as the helmet's build quality.
Measure head circumference with a soft tape, about an inch above the eyebrows, and check it against the size range printed on the helmet's packaging — most helmets come in S/M/L bands with an adjustable dial to fine-tune within that range. The helmet should sit level, about two finger-widths above the eyebrows, not tilted back off the forehead.
Once positioned, the side straps should form a V just below each ear, and the chin strap should be snug enough that only one or two fingers fit between it and the chin. If a helmet shifts noticeably when you shake your head side to side with the strap fastened, it's either the wrong size or needs further adjustment before it's doing its job.