The exact headset size you'll need is listed in a format called S.H.I.S., which is a fancy acronym for Standardized Headset Identification System.
We list S.H.I.S. in our Service Parts Reference documents, available under downloads on your bike's page on Norco.com.
For the current and previous model year bikes, you'll find your bike listed under the headings at the top of our page. For example, if you're looking for a current year Sight Carbon, you'll find this under Mountain, All-Mountain, Sight Carbon. Once you've found your specific model, scroll down to Warranty and Support and open up the Service Parts Reference document.
To find a Small Parts Reference for an older model, you'll need to locate your bike in our Bike Archives.
Once you've located your bike, just scroll down to the downloads section, and open the available reference document.
Another great resource for finding S.H.I.S. is Cane Creek's headset fit finder:
If you're interested in learning a little bit more about S.H.I.S. our friends at Park Tool also have a great article: https://www.parktool.com/blog/repair-help/standardized-headset-identification-system