Lots of good advice. I use a 14 or 16 inch round hoop BUT when I get to the edges I have a half circle hoop. It is hinged to open up and has a strip of fabric that you can pin the straight edge of your quilt to. I will try to find one online and post the details. I have had this half hoop for a long time so don't know for sure if they are still available.
Look at this site and you will see what I am talking about.
http://www.amazon.com/s?rh=n%3A262646011