I finally got my pump in the mail from insurance. Yay!! I pumped 2 ozs from one side, since I had just fed R. It was about 20 mins. Is that good?

I looked on the Medela website and I see the chart for the breast shields. I am confused as to how to know what size I need. The 24 is good but then at the same time my nipple looked pinched after I stopped pumping, so does that mean it is too tight?