I've not found out and I have tonnes of clothes lol!!!
Kid Colour (online website) is really good for fun/colourful gender neutral clothes, although everyone tends to associate pink as being a girl, dress your baby in any colour other than pink and they are automatically assumed to be a boy
Anyway, I'm quite happy to dress a boy in pink so I've happily bought all colours and any clothing I liked lol.
Sizing is frustrating. I have bought mostly Frugi clothing as they use organic cotton and they are 'cut for cloth' (they are roomier around the bottom for a cloth nappy which is a bit bulkier than a disposable, and I'm exclusively using cloth nappies).
However they generally come up as big all over.
I have 0-3m vests by Frugi that are the exact same size as one I bought off "kid colour" that claims to be "9months"
Also I have 6 babygrows all the exact same size, 2 are Frugi "newborn", two are Jojo maman bebe "tiny baby" and 2 are "blooming tots" size "0-3"
Generally with my last babies I found Next to be big (my babies were both small at birth but right up to 3/4yrs old they only fit in one size as they were leaving that age range, eg at 6 months they finally fit 3-6m clothes, at 9 months they went into 6-9 and so on lol)
Also I got "tiny baby" clothes for them from Boots and Next and they fit into them until around a pound heavier than the weight (like we had some that said 'up to 7lb' that the fit til they were over 8lbs. My eldest weighed 5lb 15oz at birth and went into some 'early baby' clothes from Boots that said "up to 5lb' and fit them til they weighed 7lb!
I think newborn vests will be fine, and any footless rompers. The only thing that might be quickly grown out of is sleepsuits/babygrows with feet, but even then I would think it would fit at least a month unless you have a huge baby.
Only other thing I've heard is that current range Next sleepsuits have small feet, a lot of people have said their baby's feet were too big (long) for the feet so maybe consider that x
Also remember babies lose a bit usually and don't get back to birth weight until around 2 weeks old (usually). So if they were 8lb born they shouldn't be 9lb until around 3/4 weeks old (if they lose some, get back to 8lb at 2 weeks, then gain around 7oz a week)