Cornrows can be any size or shape really, it depends on the sectioning you take. Synthetic hair isn't always necessary, unless you have fine hair or its a little on the short side. For example, I add synthetic hair when I'm doing a weave track all the time cause it gives you grip and a solid foundation.
It might not be necessary for you though. I usually get the yaki synthetic hair when I'm doing this cause it's got a strong texture in it and helps hold the braid in place.
Just have a consultation with a salon that specializes in braids and natural hairstyles and they should be able to help you out and let you know what you need.