My son didn't ever really fit newborn clothes either. Sizes can vary wildly between brands but in general 0-3months is the same size as 3 months, 3-6 is the same as 6, and so forth. Babies don't need tons of beautiful clothes especially considering how fast they grow. Onesies and sleeping gowns with elasticized bottoms (to keep legs warm but make diaper changes easy) are most important in those first couple of months. Basic is best, basic is easy, basic isn't fancy but all the pieces match each other.
My daugter was 7 even at birth but was petite and was in newborn clothes for a month and a half. She couldve left the hospital in premie clothes though.
Oh ok cause I got a 5 piece set of newborn onesies like the basic white ones and she said that was enough and should just start buying 3m and up lol
My son never wore newborn he was born at 8lb 15oz he started in 0 to 3 or 3 month I find that different brands fit differently just like womens jeans no set dimensions but just remember baby will only grow so u cant go wrong with a slightly bigger outfit