The best fitting nappies I had for my ds2 who was prem was xs swaddlebees and size 0 kissaluvs. The tiniest wraps I had were greenabubs soakers that came from lovely ladies on here. Next best after that was a swaddlebees merino wrap and a sugarpeas merino wrap both which were labelled small but the fit was more like xs/0.
ds2 was 34 week 5lb/10 (so big for prem) but he lost weight.
The swaddlebees and kissaluvs I had would have fit a smaller baby.
Diddydiapers and bimbles I found too big at first. I had a newborn puddlekins but it was too big for ds2 at first, in fact it drowned him not quite up to his armpits but you get the picture.