My baby is 9 months and only 17lbs so I ordered the medium (which is supposed to fit 18 pounds on the low end) and I find the top snap nearly impossible to snap around her belly because it is so tight (and I find the small disposable swim diapers to be pretty baggy and large on her).  I am going to order a large and hope for a better fit around her waist.  So, definitely order bigger than you think you need.
That aside, I really like this diaper.  It doesn't hold as much water getting out of the pool as the disposable ones do, so the towels aren't soaking wet from her butt still leaking water 5 minutes later.  It seems to be comfy and most importantly I am much more confident that if we ever have an accident in the pool this trim diaper holds a much better chance of containing it than the floppy oversized disposable ones.