Working with small holes, thin wires and small components, I use 0.4mm, it allows you to better see where the solder is going and control the amount of solder you're applying. It works fine for larger size electronics as well, though it never hurts to have some 1mm on hand.
So, something around 0.4mm = 0.016
DAN20, 1mm (0.04) solder on the left, 0.4mm on the right.