Sainsbury's try and sell anybody not the size of a stick a third less food than they actually need for the job. Say you're cooking for three. This also appears to apply to height - taking Andrew with me I get given a smaller fish than if I go on my own, every time. They are very odd there.

Next time I think I'll just quote a weight (since, having noted the weight of the fish I was given, I now have one data point) and not attempt to involve the fishmonger's brain at all. That sounds a lot safer to me :-)

Or, go to the proper fishmongers on the market, where they will actually give you good advice about what fish to choose, and you'll get better quality stuff too.