On the advice of a friend, I purchased the 6-12 months size for my small 4 month old. It's big on him but it still works well - I can feel that he is warm inside when I take it off. The suit has wide velcro fastenings so you can pull it more closed initially, and then losen it as the baby grows, saving you buying a smaller suit which they would grow out of quickly.
My baby seemed comfortable in the suit even from the first outting and it certainly does help you to keep a good grip on a wriggly slippery baby.
I would give it a 5 star rating but I don't love the logos all over it. In addition to the dolphin logo on the front that is clearly visible in the photo, there is also another big logo on the back which says "my babywarmer from konfidence" in bright yellow and white. There are also some random bubbles in a raised pattern on the front lower right corner. One of these would be fine but 3 seems like too much on a tiny little wetsuit.