As big as they want i guess.
The largest species can reach the size of a soccer ball. Most species, however, are only going to be about 2 inches.
They vary in size, I've seen ones smaller than the tip of a pinky, up to ones almost as big as a plate.
Catch a Paras or Parasect, it might be an Hold item, or maybe it's a small mushroom instead of a big one.
The giant squid can reach a length of up to 18 meters. They probably get so big because larger creatures generally have fewer predators.
In television series Family Guy, there is a giant chicken known as the Big Chicken.