One-size diapers adjusts as your baby grows. Most one-size diapers will fit approximately from 8-35lbs. All types of diapers such as fitted, prefolds, pocket and all-In-one diapers can be one-size.
The advantage of one-size diapers is that you will only have to purchase one set of diapers from birth to potty training saving you some costs. The disadvantage is that you will not get the same trim fit as a sized diaper as well as the one-size diaper will experience more wear and tear than a sized diaper possibly ot lasting as long for other children.