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: