I think its just a pad one size fits all
Honestly I use what ever shield they have all worked fine for me my son is almost 2 and still breastfeeds with a shield 
but I never used a certain size
I would have a lactation specialist help you with that to make sure you get good fit and tips
This shows the different sizes and how to measure.
They just gave me them in the hospital. I had no idea there were sizes
There are printable nipple rulers online. I used them to find the right flange size for my pump and it helped so much.
Printable nipple ruler if u are in between sizes try the bigger size first if that feels uncomfortable try the smaller one… im in between medium and small size on my right boob and a small on my left.
Lactation specialist can help you determine the best size
Definitely go to a lactation consultant, not only to be fitted properly but to see if there’s any way to not have to use them. Some babies don’t want to nurse without a shield once they get used to it.
There are a few different shapes to try. Just buy a few and see what works for you. I never knew there was an issue with sizing. The reason for the nipple shield is having inverted short nipples. Whichever you buy should just fit. They do have different size holes in them for younger and older babies
Check out this page!! The mom who runs the page - Kaylanna Hale - she does sizing and helped me out, along with tons of other moms!
Breast shields work wonders, did for me anyways. I didn’t measure, just bought 1st one I seen lol
Breast shield helped out so much. I had to use a shield because my child had trouble latching since my nipples were small. I got my measured by the lactation consultant.
It helped me alot but honestly I didn’t know at that time it has different sizes 
i just bought one and used it somehow
I bought mine too small and my lactation consultant told me to size it to your nipple not baby’s mouth.
I had to go up a size!
I also, used a little lanolin to help it stay on and seal a little better!
I have one and it’s the best invention ever. You’ll need to measure your nipple in mm.