My 7 year old daughter is 45" tall and 43 pounds. (short and skinny for her age).  Based on the measurements of the O'Neill rash guard, we ordered a size 4 for her. It fits her perfect! It is skin tight, just like it is supposed to be.

It rides up a tiny bit at the waist in the water, but I just make sure she has sunscreen on there or is wearing a one piece bathing suit under the swim shirt.

The neck hole is rather small and a parent has to help her put it on and off because of this issue, but if the neck hole was any bigger, there would be a big gap around her neck, needing more sunscreen! But the shirt is a GREAT fit, perfect sizing according to the size charts and I would definitely buy this product again.

We have a blue size 6 for her brother (49" tall, 58 pounds) and it just finished it's second swimming season. It is now pilly across the front and getting really thin, though still protects from the sun. The pills are probably from getting in and out of the pool. It also balloons up in the water when it's wet now, but I don't think that is bad for lasting two whole summer seasons in the chlorine.

I care for them by rinsing them on a gentle no soap rinse in my washing machine (by themselves) and then I lay them flat to dry. We are very pleased with this product and would definitely buy again! The only other experience I have with long sleeve rash guards are from lands end, and O'Neill fits way better. Land's End's are looser from the get go, which is not appropriate for a rash guard.