S-M fits hips 65-95cm (this style is printed as seen and packaged in a green resealable bag)
M-L fits hips 75-110cm
L-XL fits hips 100-140cm
For C-Section Mamas, it is best to get a little bigger if you are between sizes so they are not too tight.
If you are after a more reusable option, check out Modibodi underwear, which comes in a range of sizes, styles and absorbencies. They have recently released a “MAXI” absorption which would be a good start for postpartum.