UPDATE: Restock!  (see the last 2 pictures)
Small ones are approx 0.8" - 2,3cm, big ones are 1.6" - 4cm (the bigger ones have the closure with a hook. All are deadstock c. 1960's-70's, with slight storage marks.
 
Deadstock original 1960’s Harvey Ball smiley pins. (Big boy size of Ø4cm). New old stock, but these might have a few scratches from being in storage all this time.
 
Harvey Ross Ball (1921 – 2001), an American graphic artist and ad man, is the earliest known designer of the Smiley. His design of the smiley came about in 1963, when he was commissioned to create a graphic to raise morale among the employees of an insurance company after a series of tough transitions. The aim was to get employees to smile while using the phone and doing other tasks.
 
“I made a circle with a smile for a mouth on yellow paper, because it was sunshiny and bright,” he later told the Associated Press.