Question: What is the smallest explosive and what size is it?

Ciarán O'Brien answered on 12 Nov 2014:

Hydrogen is a very explosive gas, and it’s also the smallest atom in the universe. It’s hard to contain in amounts that will do any damage though, and the mechanism you’d need to contain and ignite it would lead to a very large bomb for all the explosion you’d get out of it.

C4 gives you way more bang for your buck. It’s soft like Plasticine so it’s easy to stick to things and mould into shape, and it’s small and easy to carry. It’s such a good explosive that armies have been using it for decades. Half a kilogram of that stuff will blow holes in tank armour.