Flange sizing???

How do I figure out my flange size? I bought a 24 MM size breast pump, and it’s way too large but I can’t figure out what size I am. When I give birth, will the doctor be able to help me figure out what size I need for my breast pump or is that something I have to figure out on my own? I’m not sure how to figure something like that out and I don’t know if the doctor will be able to help me that way I can be able to pump and feed like I’m planning to. I’m also due in less than a week now so I’m panicking