We enthusiastically bought lots of onesies, pajamas and bodysuits for newborns, only to find that our baby turned out to be quite long (56 cm/22 in) and thus the newborn-sized clothes were actually too short for her and we went straight to dressing her in outfits for babies 1-2 months of age. This was a big lesson learned for me, to rather get only a couple of newborn-sized onesies and bodies and only when your doctor gives you a clear general tendency for the size of your baby in the last pregnancy weeks or when the baby is actually there and you know what fits him/her, is it safe to order more newborn options online and have them delivered, rather than piling up lots of stuff that might not even fit.
That being said, there is no standard newborn size when it comes to clothes (so exactly the same challenge as with clothes for us grown-ups). Every company uses its own measurement. So while our daughter was definitely too big for newborn outfits from the vast majority of companies, everything we bought at Gymboree in the US was magically a perfect fit. In Germany, baby outfits are mostly measured in centimeters unlike in the US where it’s newborn/1-3 month/3-6 month, etc. So I recommend erring on the safe side and rather than getting the small size (50 cm/19,5 in, newborn), opt for the next one up (56 cm/22 in, 1-2 month). However: don’t forget the average newborn is 51-52 cm/20 in.
Also, don’t forget that your baby will only wear each of its outfits for a very limited amount of time, as it will outgrow even the cutest onesie within 2-3 month. Hence, a great resource for clothing is second-hand outfits, either through family and friends or through online second-hand websites and the numerous baby flea markets out there. Maybe get one or two cute key pieces new (as we all know, it is soooo much fun browsing a store full of baby clothes) and get the rest as hand-me-downs or second-hand. Especially since it can actually bring friends and family great satisfaction to know that another kid will enjoy wearing an outfit that already looked very adorable on their own baby.
Moreover, with everything you get, keep in mind that you will go through 6 to 9 diaper changes every day. That means you will partially undress and dress your baby 6 to 9 times a day. Sometimes you will go through those motions with a perfectly calm and content baby, other times your little bundle of joy will seem less joyful and scream at the top of her lungs and you want to be quick. So convenience is key when it comes to the clothing you pick. Hence, the smaller the number of tiny buttons you need to fasten, the better, as this all takes time and is fiddly. If you can get outfits with a zipper, do yourself a favor and get those, as they are vastly superior to buttoned ones in terms of convenience. It is much faster, which is especially essential for the nighttime diaper changes when you are still half asleep.
Keep the seasons in mind! So if your baby is born in June your starter sleeping bag should be a light one and you want to have short sleeved outfits handy for starters. If you shop ahead for the outfits for the first year, do your math, does your baby need warmer winter clothes when she/he is 6 month old or will it be spring/summer/fall by then? We got the cutest warm woolen hat as a birthday present for our daughter that is for kids 3-6 month old. So it would fit her right at the height of summer and since we have no plans to take her on a hike on a glacier, she will thus probably never use it. It’s a bit trickier if your little munchkin is born in spring or fall, but for anything your newborn will wear that is NOT for outside/walks with the stroller, a long sleeve light version probably suffices unless you are strictly against heating your home.
Regarding sleeping bags. One per size is enough. If that one needs washing simply use the one the next size up. I recommend getting the first three sleeping bags, again on of each size (newborn/50cm, 1-2 month/56cm and 3-6 month/68cm), then you are all set for the first couple of month. Here size is less of an issue anyways as your mini-me will actually even enjoy a more compact bag as it reminds him/her of the tightness of the womb, which is makes your baby feel calm and secure.