I ofund several patterns online and most of them tell you how many stitches you need to cast on for each ripple. You would need to knit a small swatch to determine the size with the needles and yarn you want to use, then decide how many repeats you will want to make.

http://www.lionbrand.com/patterns/khs-catchTheWaveAfghan.html

http://www.knitting-crochet.com/fea.html

http://cache.lionbrand.com/patterns/1181-K.html

http://www.coatsandclark.com/Crafts/Knitting/Projects/AfghansThrows/LT1542+Dreamy+Ripple+Knit+Afghan.htm

http://www.coatsandclark.com/Crafts/Knitting/Projects/AfghansThrows/LW1426+Feathery+Fans.htm

Thanks so much. The links were just what I needed.

My pleasure.