Anyone know how to figure out the size of the pump you need?

You need to know your flange size. Most pumps come with size 24mm and 28mm. They are very big. You can order flange inserts to make them smaller to fit you well. You’ll need to measure the diameter of each nipple to get the size.
*just your nipple, not the surrounding,

Yes you can order a flange sizing kit/nipple measurer on Amazon! With my first I did trail and error which could’ve been avoided if I just measured my nipples lol. But like Barbara said most pumps do come in 24-28mm which is the standard so you’ll most likely need an insert.

There’s so many Facebook groups and I was actually sized by lactation specialist through Facebook message and it helped SO much. There’s also pics if you google how to determine flange size!

I used this and it was accurate for me

https://breezybabies.com/products/free-nipple-measuring-tool-find-your-flange-size

A lactation specialist !! I ended up needing a size 15 flange

Wow I didn’t even know this! Thank you for asking this question!