Neither of mine wore 0000 for more than a day (e.g. put them in first day and were too small) both over 4kg bubs.

It also depends on the item - my DS is like Santosha's and he isn't six months and he is in some 1's - his body is very long so if anything does up under crotch he needs really big sizes, tops and jackets etc he could still wear 00. Jackets in particular seem sized big and with long arms (DD wore a 000 jacket whole of her first year). Brands really vary I have a short sleeved onesie from GAP which is a hand me down and is size 18M to 24M and it fits my 6 month old (it must have shrunk a little).

Stuff with feet in them have a more limited shelf life (I cut them off sometimes (the feet off the suits that is :-)).

I don't buy much but am always in a rush it seems to get some wear out of the stuff people give us as sizing is so difficult - or we get bought bigger stuff but then is the wrong season.

If I had to pick a size I would say 00.