from my experience people tend to buy you bigger sizes not newborn - most of the stuff I was given was 00 and 0.
DS went straight in to 000, and I had always expected to have a larger bub (he was 10lb4oz) so I'd never bought anything smaller than that anyway. If you need a smaller size you can always go get it but if you buy too small you may not get to use it at all.
He was in 00 by 8 weeks, went to an 0 by about 4-5 months then stayed in size 0 until he was about 14 months old.

I definitely agree with the seasons thing, though, at least in Melb, I find things like hoodies can be worn on all but the hottest of summer days, and you can always put a long sleeved top under a tshirt if its cold. Its mainly shorts and jumpers (wool ones) that are seasonal.