I got my snap press back when they weren't available in Australia, so got it from the US and waited months for slow postage (due to the weight it would have cost the earth to get here faster). You girls are so lucky to get them here now. Definitely size 20 for nappies - I used 16's on all my small npockets last time (all 24!!!) and found that if he kicked his legs too much they popped open. So I got 18's for the rest - in the US where I bought my snap press they sold long prong 18's which have an 18 cap and a 20 working piece (stud/socket). Great to get through the thicknesses and not too bulky, but there isn't alot of difference between the 20's and 18's as I have some coloured snaps from snaps Australia in the 20 size and they still fit in my 18 cap die, but just get a little stuck and need to be tugged slightly to pull out after snapping. Not enough of a problem to buy a new cap die, unless I bought loads more 20 snaps.