How to gauge flange size when buying a pump online? Does having a big breast mean u need big flanges? Any guideline to follow?

When purchasing flanges, it is all about a comfortable fit. It is an inexact science and apparently has nothing to do with the size of your breasts but rather the size of your nipples. Where do you intend to buy your flanges and/or breast pump from? Medela has a guide on choosing the appropriate size flange for different sized nipples. You can read about it here: http://www.medelabreastfeedingus.com/tips-and-solutions/13/choosing-your-personalfit-breastshield-size