joe'smom, I cut my pieces larger and then trim down. If you are getting one side longer, then you're either stretching to much or not enough. If you clip the seam first, it gets real easy to over stretch, and if you don't clip enough it can be really easy to not stretch enough. I always put the "outer circle edge down and the inner circle edge on top.
It really helps if you are sewing on a straight stitch foot so you don't have a lot of feed dog trying to hold the fabric straight when you need to be turning it. I also like to shorten up my stitch length - as the smaller stitches make going around the curve easier.