I'm such a nerd
I spent a good few minutes wandering around the house looking for stuff with grommets on it and measuring them
to see what size would be most useful. Most of the ones on kid's clothing are around 7-8mm internal diameter; while the ones on DH's hoodies are more like 1 cm internal diameter. So...I'm thinking of ordering a 7-8 mm one, cos I think it'd get the most useage here - the larger ones look a bit clunky IMO.
Might as well take advantage of the good exchange rate at the moment and go a bit wild