In reply to Georgina Lamb:
I've got a 23" waist and ended up buying an Arcteryx R280 in XS and this has been fine (maybe try and find one on offer, as they're a bit pricey, but the advantage is that they are very compact). I also tried a DMM harness (can't remember which one) and it would have been suitable too.
I found other brands - Wild Country, Black Diamond, and Petzl - were just way too big.