I normally wear a large to XL in women's clothing and the L/XL/XXL fit perfectly.  I think the smaller size would have worked too since this comes with velcro on the back to make it much wider or smaller and an extra zippered strip for the front to achieve a perfect fit there.  It does take an extra minute or two to set up but it has allowed me to read/use my laptop instead of sitting there holding two pumps.  Wish I had it sooner.