We grew a new one 'Krypton', which have settled well in a block of around 30 (the number of seeds in the packet is small).
They've grown very tall, which is great for a small plot, and when harvested you can use nearly a couple of ft of veg, which can't be bad!
As for size, the smallest is around 3/4" and the largest 1.5", but the length cancels out the width, as we like some of the green showing as well as the white underground!
I actually thought they were a late leek, but have read elsewhere that they aren't, but they're standing well so I'm very happy!