Getting the right size of nappy is really important to keep your baby comfortable and to prevent leaks. Nappies sizes relate to the weight of your baby. As babies come in different shapes and sizes, and to give some flexibility as your baby develops, you'll see there is a slight overlap between sizes.
Here's a handy guide for Pura nappies:
Size 1: newborn 2 - 5 kg / 4 - 11 lbs
Size 2: mini 3 - 6 kg / 6 - 13 lbs
Size 3: midi 6-10 kg / 13 - 22 lbs
Size 4: maxi 7-18 kg / 15 - 40 lbs
Size 5: junior 11 - 25 kg / 24 - 55 lbs
Size 6: xl 15+ kg / 33+ lbs
Check that it's fastened properly, not too tight but not too loose, by testing that you can fit two fingers inside the waistband. If the nappy doesn't cover your babies bottom, regularly becomes soaking wet, or you see red marks or signs of chafing, this may mean it's too small for your baby.