Our son is almost 5 and, per other parent's suggestions, we went with the 6/7 size and have not regretted it one bit.  Our son goes to a Montessori nature preschool in the afternoon and is outside in the elements.  This is perfect on the days that it has been 48 degrees and raining, plus there is still snow on the ground.  I have layered him in a long sleeve shirt, sweatshirt, puffy jacket, long johns, pants, snow pants and then this rain suit and the kid stays warm and BONE DRY.  The teachers at the school were so impressed with the suit (we live in a small town) that they took a picture of the logo so they could recommend it to other parents.  Other kids have rain suits, but they're not waterproof and they end up getting soaked.  I've washed it many times and it's still holding up great...just wash it inside out, on gentle/cold and dry on gentle too.