When it comes to choosing your cloth diaper, sizing is an important decision. Some diapers come in several sizes. Some come in 1-3 sizes. If you have the option of a diaper service and are using it then changing sizes regularly will have no financial impact.
Infact prefolds which were the first sized diaper were invented by a diaper service and became the Curity prefolds (the ones my mom used). So having 5-6 sizes of diaper that a baby would go through till potty trained was a practical choice.
Today the majority if us are washing our own diapers and own our own diapers. That would be 4-6 diaper stashes. If you can afford it and want to then go for it.
There are ones that are 1 size and that's from 7 or 10 lbs to 35 lbs. I find such diapers do not truly fit babies till about 12-16 lbs (depending on shape of baby). This is one case where I doubt its a good design for anyone, unless they are choosing cloth when baby is a little older and already that size.
Some diapers come in 2 sizes (infant and toddler) and sometimes a third option (Preemie or newborn). I find this a sensible choice for most people. I have always choosen diapers that come in infant and toddler. They tend not to fit my babies in the first 4-10 days. I'm ok with that. (I bought a pack of newborn sposies for the time being with babies 1-4. With baby 5 the kushies basic adjusted down better.) The infant size ms tends to be 7 or 10lbs to 20 or 22lbs. The toddler sizes usually goes up to 30 or 40lbs.
Below I've included a link to a site that has size charts for various diapers (scroll down a little). Look at all the options and consider how often you plan to buy another stash. If your top few choices in diapers are all multiply sized, see if they carry a size similar to 10-22lbs (infant) and 20 or 25lbs to 35lbs (toddler), This will work for buying just two sizes. Some multiply sized diapers have overlaps in sizing that will allow you to pick just two. If one of your top diaper picks comes in infant and toddler I'd go with that. In the end you know what your finances allow and what you think is sensible in your circumstances.
If your using prefolds you can fold down to a smaller baby more easily. Prefolds are just more.flexible. And so I'd suggest the same when choosing sizes. I will be making my prefolds in an infant and a toddler size. This time I will be able to fold just a bit smaller for those first few days. (i bought fitteds before and just used prefolds at my moms). Covers can get in the way of the cord in the first few days. This does not mean you need a special cover. Babies barely pee or poop till your milk comes in. Cords and milk happen very close together. And until the milk comes you wont need a cover.
Frankly if I was buying this time and just about to begin my diaper choice search I would only check out diapers that came in the sizes I liked. It would narrow down options to look at. Make it all a little more simple.
MAKING A DIAPER FIT
Diaper is to big:
If you want to make that diaper fit baby when its a couple lbs to big there are options.
1. If its a prefold just fold a little smaller.
2. If its fitted add a few extra liners and tuck it all as tight as possible. You'll need the liners to fill in at the leg. This will not be uncomfortable for baby. The froggy legs are actually comfy for infants and good for their hips.
* a fitted diaper of 2-4 layers will adjust to a smaller baby easier then one of 6-8 layers.
Diaper is to small:
Sadly no solution here that im aware of. A diaper that is to small will leak more.
What we can learn:
1. All diapers work, but not necessarily all size options.
2. Prefolds are more adjustable when it comes to sizing
3. Thinner diapers with less layers are more adjustable then a thicker.diaper.
4. You dont need diaper covers till your milk comes in.