Can anyone help me find the right breast pump flange size?

Hello 👋🏼 I’m a first time mom and 29 weeks pregnant. I love to be prepared and I live in a small town with bigger cities an 1+ away. During the pandemic I worry about getting things shipped /mailed in a decent amount of time so I was trying to figure out my flange size or roughly close to it. Will my nipples change in size a lot the next 10 weeks? I have a spectra pump and the 24 mm flange just looks too big. My nipple doesn’t even go into the tunnel. I measured my nipple and it’s 15mm (I looked up how to measure). Should I order 20 or 21 mm flanges or just wait until the baby comes and pray I don’t have to wait a month to get the right size? Thank you for your help!! I’m clueless!