If you were going to do a wrap style soaker (like the BBH one) and have velcro closures on it then you would knit it big and then wash it in the machine and dry it in the dryer before sewing your velcro onto it. So for a medium sized soaker you would use a large not felting pattern and then shrink it. Or in my case they were 8kg+ soakers and they accidentally went in the dryer, so now they are NB or small size.