Ok, previous to our last trip this past Feb I read on another thread that you had to pick the smallest, ugliest and hairiest of the shells.
So, I surprised DD5 and DW with a 'pick' for each one. DD chose thd one that she liked the most, but DW followed the Dis advice (which I told her), and she got an 8mm, which is pretty big. DD5's turned out to be a 7mm.
I've read that picking that way you get the best results.
Good luck!
Mx