We didn't use prefolds but I know that the Green Mountain Diapers prefolds are super popular so I'd probably get similar sizes or make something custom to suit your needs. It seems like the regular infant/premium prefolds are rectangular and many people find them too long (so you have to fold them down adding extra bulk) and by the time you need that extra length they are too narrow and won't wrap around the baby well anymore. So the more squared shape of the GMD prefolds seems to be what makes the difference. If you are making your own you could always do something between their newborn and small sizes if you want, or adjust it depending on the expected size of the baby.
Here's their size chart:
Don't plan on using the one size diapers for a while, I think most normally won't fit till at least 10 lbs, depending on baby's build, and don't accommodate the healing umbilical cord. So unless you are planning to do some sposies at first, newborn sizes are a good idea and especially prefolds are versatile for diaper inserts/doublers or even burp cloths after.