There is no universal standard for the size of fun. This so called "Fun Size" takes many forms. Conveniently, all forms that I'm aware of are contained within the realm of candy.
The most common form is of a bar about 1 1/2 inches square and 1 inch thick. These can be found in the following candies:
Of course, there are candies that, due to their form, cannot possibly be
congealed into a bar without losing some of their tiny
candy magic in the
process. These all contain about 20 candies in every
fun size bag:
And there are a few
unimaginative companies that just give half of what is in their normal packages: