This product should come in the box with the pump!  I have both sizes.  I first bought a large as that was all that was available and I NEEDED to stop holding the pumps, my back was so sore.  I use the tighest velco setting on the back and the top is a bit loose when I am not "full".  This turns out to be ok though.  When pumping you must lean forward to the milk doesn't pool up in the flanges, when the top is loose it sags a bit and the pumps angle downward.  I bought a small too, but the breast holes are too close together.  I use the expander that came with the large - no optional front center expanded comes with the small.  I am 5'5 136 lbs 2 weeks postpartum.  I am a 34 around the rib cage but have large, wide set breasts.  I have to be a DDD or E cup, or more.  I almost bust out of the size large bravado nursing bra.

Great product, a must have.