When I first began cloth diapering, I bought only bought diapers that I could a adjust to multiple sizes. This was handy because I was not sure if my two year old or even my three year old would be fully potty trained by the time baby number three would arrive. I used Fuzzi Bunz one size cloth diapers, where one adjusts the diaper with the elastic inside the diaper. The benefits to this kind of diaper were there were several size settings, leg holes could be snug, and I was able to fit them on an eight pound baby and my three year old. However, the down side is these diapers don't last for years and years. At least mine did not. When one buys diapers for different sizes of babies there are more diapers being used for a shorter amount of time. I know people who have used their Fuzzi Bunz for six different kids. I had other problems with the Fuzzi Bunz, not related to them wearing out, but if you buy a diaper to fit you child for two plus years, you cannot realistically plan on making work for the next baby. My Flip diapers were great for one child, but pretty trashed by the end of two years.
I started to make diapers sized to fit my baby. For me this made sense because I did not want to invest in a snap press right away and I could make so many more diapers and still spend less money than buying 20-24 one size cloth diapers. I used old clothing for the material in some of my diapers, and bought nice fabrics for others. Old prefolds became AIO diapers. It is nice to not have to change the length of the diaper elastic or snap diapers to be the right size and shape, even though it is not hard to change diaper settings.
For RubyRumps, I decided to make three sizes of diapers. People don't always want to start cloth diapers the second a baby is born. Sometime one wants to start around 6 months old. Most of the cloth diaper size changes happen in the first six months where baby is doubling his or her birth weight. Then I have noticed on my own children I can get to size L cloth diaper, but then once baby is seriously crawling and then walking, a size M may fit again. I would say that my youngest daughter is in size L or M at this point because I like the fact that the L has a bit more coverage, but she fits nicely into a M too. She's around 16 pounds, maybe 17 by now, so she is on the small and slim side for a one year old. My two year old fits into a size L more perfectly. I can even get my skinny three year old into a size L diaper. For special requests I may make a larger diapers, but size L works for a lot of different weights and shapes.
Recently I gave in and started to make some size newborn cloth diapers. I have cloth diapered a newborn and these diapers last two to three months depending on babies birth weight. They look so cute and it is nice to have a perfect fitting cloth diapers for baby the first day. It can get a bit expensive to buy tons of cloth diapers for a newborn, but one can also consider part time cloth diapering. Newborns get through so many diapers it is nice to have some cloth ones. Also, most one size diapers will not fit on a newborn baby. They work well for eight pounds and up. Most of my babies have been in the five to six pound range. I have never tried to put a one size diaper on them, but even a newborn paper diapers has been too big for some of my babies.
Sizing diapers can be confusing and frustrating, but really think about it like this: Size small is similar to a 1 or 2 size paper diapers, Size medium is a 3, Size large is a 4 or 5 paper diapers, and upon special request my newborn diapers are about the same size as a newborn paper diaper.