Need Help with recovering couch cushions Sewing Machines Parts and Sales in Los Angeles

My boyfriend’s parents asked me to recover their couch cushions. I know how to do it, that’s not the hard part. The issue is that I don’t know how much fabric to tell them to order. I’m not sure how upholstery fabric comes, or what sizes. I’m not even sure where to direct them for purchasing the fabric. I just want to make sure I’m doing this right. Help?