Nuliie 3-in-1 Flange Sizing Kit 13/15/17/19/21mm for 24mm Electric Double Breast Pump Flange/Shield, 10PCS Silicone Flange Inserts to Boost Pumping Efficiency & Comfort, BPA-Free Essential for Moms

Seconding - OP I recommend getting a flange sizing kit that comes with a bunch of flange inserts - like this It’ll help you size yourself on correct flange size and also keep the ruler and other inserts jic because nipple size can change over time.